BUG: Added checks for NaN in __call__ of EngFormatter
- [x] closes #11981 
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Closes #11981

Updated the v0.18.2 document

In the function `__call__` of EngFormatter check if decimal input is NaN.

once you have made the changes and pushed, ping on green.
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13124?src=pr) is **84.14%**

> Merging [#13124](https://codecov.io/gh/pydata/pandas/pull/13124?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **-<.01%**

``` diff
@@             master     #13124   diff @@
==========================================
  Files           137        137          
  Lines         50287      50289     +2   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42312      42313     +1   
- Misses         7975       7976     +1   
  Partials          0          0          
```
1. File `...das/tseries/index.py` (not in diff) was modified. [more](https://codecov.io/gh/pydata/pandas/commit/d7315dd64adfd577abe0dccf2f371aa2d5b6d80c/changes?src=pr#70616E6461732F747365726965732F696E6465782E7079) 
   - Misses `+1` 
   - Partials `0` 
   - Hits `-1`

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [43989fd...d7315dd](https://codecov.io/gh/pydata/pandas/compare/43989fd7cb9917e885c2b55a172c4f9f3838d59d...d7315dd64adfd577abe0dccf2f371aa2d5b6d80c)
 @jreback the Travis-ci build job 19538.5 has failed for Python 3.5, as except with comma is not valid in Python 3. But, surprisingly the tests have passed for Python 3.4.

```
File "/usr/share/google/boto/boto_plugins/compute_auth.py", line 64

    except (urllib2.URLError, urllib2.HTTPError, IOError), e:

                                                         ^
SyntaxError: invalid syntax
```

Is this Pandas error or testing framework error. Should I run the Travis again.
 thanks @yaduart 
 As I am a newbie here, I am not sure of the processes. It would help me understand how to close the pull request, if you could tell how you decided to close the pull request as the Travis-ci checks were failing, or did you run them again, or was the Travis-ci bypassed.
 @yaduart no, I merged it via https://github.com/pydata/pandas/commit/d0734ba4d0f4c228110dc3974943ce4ec2adeea4

it is a slightly edited commit. When I merged it closed the PR and the issue.

thanks for the contribution.
 If I had closed it for another reason (e.g. not accepting) or something would always write something.
 Why was the travis-ci check failing earlier?
 unrelated, we picked up a broken `boto` version (not sure why it was released), and were importing it on main pandas import (which is wrong, but that's how it was). 
 Thanks for clarifying. Cheers
