BUG: rank with +-inf, #6945
The bug is caused by replacing nan with positive infinite or negative infinite values. As a result, when keep_na is true, both real nan and positive/negative infinite will be assigned nan as the rank. The solution is to keep track of the real nan value when generate the nan mask. 

- [x] closes #6945 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

tests! # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17903?src=pr&el=h1) Report
> Merging [#17903](https://codecov.io/gh/pandas-dev/pandas/pull/17903?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0e168188811677f9de72a6a5b97253e551b6b04a?src=pr&el=desc) will **increase** coverage by `0.11%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17903/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17903?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17903      +/-   ##
==========================================
+ Coverage   91.46%   91.58%   +0.11%     
==========================================
  Files         157      155       -2     
  Lines       51439    51255     -184     
==========================================
- Hits        47051    46941     -110     
+ Misses       4388     4314      -74
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.44% <ø> (+0.12%)` | :arrow_up: |
| #single | `40.67% <ø> (-0.05%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17903?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17903/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/17903/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `82.01% <0%> (-0.17%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17903/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |
| [pandas/io/json/normalize.py](https://codecov.io/gh/pandas-dev/pandas/pull/17903/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2pzb24vbm9ybWFsaXplLnB5) | `96.87% <0%> (-0.07%)` | :arrow_down: |
| [pandas/core/tools/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17903/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvdG9vbHMvZGF0ZXRpbWVzLnB5) | `84.43% <0%> (-0.06%)` | :arrow_down: |
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/17903/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.55% <0%> (-0.05%)` | :arrow_down: |
| [pandas/core/internals.py](https://codecov.io/gh/pandas-dev/pandas/pull/17903/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzLnB5) | `94.44% <0%> (-0.04%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17903/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.68% <0%> (-0.01%)` | :arrow_down: |
| [pandas/compat/pickle\_compat.py](https://codecov.io/gh/pandas-dev/pandas/pull/17903/diff?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9waWNrbGVfY29tcGF0LnB5) | `75.6% <0%> (ø)` | :arrow_up: |
| [pandas/core/sparse/api.py](https://codecov.io/gh/pandas-dev/pandas/pull/17903/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3BhcnNlL2FwaS5weQ==) | `100% <0%> (ø)` | :arrow_up: |
| ... and [9 more](https://codecov.io/gh/pandas-dev/pandas/pull/17903/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17903?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/17903?src=pr&el=footer). Last update [0e16818...987fb9d](https://codecov.io/gh/pandas-dev/pandas/pull/17903?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Hello @peterpanmj! Thanks for updating the PR.





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

##### Comment last updated on December 04, 2017 at 15:42 Hours UTC rebase as well as the missing value support was moved around a bit (e.g. changes to infinity w/o nan) thanks @peterpanmj ! nice patch & very responsive!