BUG: Fixed 19497 - previously, renaming an index changed its type if …
Fixes bug GH19497 - previously, renaming an index with tuples changed its type (i.e. from Index to MultiIndex).

- [x] closes #19497 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21029?src=pr&el=h1) Report
> Merging [#21029](https://codecov.io/gh/pandas-dev/pandas/pull/21029?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a5c02d5c4225a90cda8ed7f328306bd9754d0f93?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/21029/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/21029?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21029      +/-   ##
==========================================
- Coverage   91.82%   91.82%   -0.01%     
==========================================
  Files         153      153              
  Lines       49502    49502              
==========================================
- Hits        45457    45455       -2     
- Misses       4045     4047       +2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.22% <100%> (-0.01%)` | :arrow_down: |
| #single | `41.88% <100%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21029?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/internals.py](https://codecov.io/gh/pandas-dev/pandas/pull/21029/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzLnB5) | `95.59% <100%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/21029/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.38% <0%> (-0.21%)` | :arrow_down: |
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/21029/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.46% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21029?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/21029?src=pr&el=footer). Last update [a5c02d5...40a53fb](https://codecov.io/gh/pandas-dev/pandas/pull/21029?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @charlie0389  My pleasure :)