BUG: Preserve name in DatetimeIndex.snap
Use self.name to construct the result of `DatetimeIndex.snap`, fix #25575.

- [x] closes #25575
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25585?src=pr&el=h1) Report
> Merging [#25585](https://codecov.io/gh/pandas-dev/pandas/pull/25585?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/74a9ae309d057d5da50717ffb18fd3dd9212fad7?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25585/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25585?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25585      +/-   ##
==========================================
+ Coverage   91.26%   91.26%   +<.01%     
==========================================
  Files         173      173              
  Lines       52966    52966              
==========================================
+ Hits        48337    48338       +1     
+ Misses       4629     4628       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.83% <100%> (ø)` | :arrow_up: |
| #single | `41.71% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25585?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/25585/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `96.41% <100%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25585/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.66% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25585?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25585?src=pr&el=footer). Last update [74a9ae3...591f47f](https://codecov.io/gh/pandas-dev/pandas/pull/25585?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25585?src=pr&el=h1) Report
> Merging [#25585](https://codecov.io/gh/pandas-dev/pandas/pull/25585?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/74a9ae309d057d5da50717ffb18fd3dd9212fad7?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25585/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25585?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25585      +/-   ##
==========================================
- Coverage   91.26%   91.25%   -0.01%     
==========================================
  Files         173      172       -1     
  Lines       52966    52963       -3     
==========================================
- Hits        48337    48330       -7     
- Misses       4629     4633       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.82% <100%> (-0.02%)` | :arrow_down: |
| #single | `41.73% <0%> (+0.02%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25585?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/25585/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `96.41% <100%> (ø)` | :arrow_up: |
| [pandas/core/panel.py](https://codecov.io/gh/pandas-dev/pandas/pull/25585/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcGFuZWwucHk=) | `35.72% <0%> (-2.85%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/25585/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `93.51% <0%> (-0.15%)` | :arrow_down: |
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/25585/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `83.53% <0%> (-0.05%)` | :arrow_down: |
| [pandas/core/config.py](https://codecov.io/gh/pandas-dev/pandas/pull/25585/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29uZmlnLnB5) | `87% <0%> (-0.05%)` | :arrow_down: |
| [pandas/io/formats/format.py](https://codecov.io/gh/pandas-dev/pandas/pull/25585/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZm9ybWF0LnB5) | `97.99% <0%> (ø)` | :arrow_up: |
| [pandas/core/arrays/sparse.py](https://codecov.io/gh/pandas-dev/pandas/pull/25585/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3NwYXJzZS5weQ==) | `92.17% <0%> (ø)` | :arrow_up: |
| [pandas/core/groupby/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/25585/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9iYXNlLnB5) | `91.83% <0%> (ø)` | :arrow_up: |
| [pandas/core/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/25585/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYmFzZS5weQ==) | `97.76% <0%> (ø)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/25585/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.79% <0%> (ø)` | :arrow_up: |
| ... and [12 more](https://codecov.io/gh/pandas-dev/pandas/pull/25585/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25585?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25585?src=pr&el=footer). Last update [74a9ae3...b169fa8](https://codecov.io/gh/pandas-dev/pandas/pull/25585?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!