Fix bug where offset.copy() != offset
Add test that will currently fail
Make some attributes into cache_readonlys

Last one for now.

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

Hello @jbrockmendel! Thanks for updating the PR.





Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on September 14, 2017 at 16:17 Hours UTC is this orthogonal on #17458 ? Yes.  They're related in spirit, but need to be fixed separately. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17452?src=pr&el=h1) Report
> Merging [#17452](https://codecov.io/gh/pandas-dev/pandas/pull/17452?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d4577911c750f2f48f760ce451d413116bed72da?src=pr&el=desc) will **increase** coverage by `0.04%`.
> The diff coverage is `98.36%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17452/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17452?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17452      +/-   ##
==========================================
+ Coverage   91.15%    91.2%   +0.04%     
==========================================
  Files         163      163              
  Lines       49587    49604      +17     
==========================================
+ Hits        45203    45243      +40     
+ Misses       4384     4361      -23
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.99% <98.36%> (+0.06%)` | :arrow_up: |
| #single | `40.19% <54.09%> (-0.12%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17452?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/17452?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `97.18% <98.36%> (+0.03%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17452?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/tseries/frequencies.py](https://codecov.io/gh/pandas-dev/pandas/pull/17452?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvZnJlcXVlbmNpZXMucHk=) | `96.11% <0%> (-0.4%)` | :arrow_down: |
| [pandas/core/sparse/array.py](https://codecov.io/gh/pandas-dev/pandas/pull/17452?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3BhcnNlL2FycmF5LnB5) | `91.3% <0%> (-0.12%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17452?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.77% <0%> (-0.05%)` | :arrow_down: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/17452?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.92% <0%> (-0.03%)` | :arrow_down: |
| [pandas/core/dtypes/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/17452?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbmNhdC5weQ==) | `98.26% <0%> (-0.03%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/17452?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `91.98% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/17452?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.28% <0%> (-0.01%)` | :arrow_down: |
| [pandas/util/\_doctools.py](https://codecov.io/gh/pandas-dev/pandas/pull/17452?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvX2RvY3Rvb2xzLnB5) | `0% <0%> (ø)` | :arrow_up: |
| ... and [20 more](https://codecov.io/gh/pandas-dev/pandas/pull/17452?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17452?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/17452?src=pr&el=footer). Last update [d457791...db8c18e](https://codecov.io/gh/pandas-dev/pandas/pull/17452?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks; normally I would ask for a whatsnew entry, but I think this is fine.