ERR: ValueError when merging on incompatible dtypes
- [ ] closes #9780
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Hello @reidy-p! Thanks for updating the PR.





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

##### Comment last updated on December 09, 2017 at 15:46 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18674?src=pr&el=h1) Report
> Merging [#18674](https://codecov.io/gh/pandas-dev/pandas/pull/18674?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/86606b2541c615821c73007dc949d2837a5af2d0?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18674/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18674?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18674      +/-   ##
==========================================
- Coverage    91.6%   91.56%   -0.05%     
==========================================
  Files         153      153              
  Lines       51273    51285      +12     
==========================================
- Hits        46970    46957      -13     
- Misses       4303     4328      +25
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.42% <100%> (-0.03%)` | :arrow_down: |
| #single | `40.67% <0%> (-0.12%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18674?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/merge.py](https://codecov.io/gh/pandas-dev/pandas/pull/18674/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZXJnZS5weQ==) | `94.2% <100%> (-0.21%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18674/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18674/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `64.78% <0%> (-1.74%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/18674/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `81.82% <0%> (-0.2%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18674/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18674?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/18674?src=pr&el=footer). Last update [86606b2...0ca53a5](https://codecov.io/gh/pandas-dev/pandas/pull/18674?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 rebase and looks good to go. thanks @reidy-p  Thanks for the fix @reidy-p and @jreback