BUG: Rolling negative window issue fix #13383

[priyankjain]

• closes Series.rolling/DataFrame.rolling don't fully check arguments, have odd behavior when used with invalid inputs #13383
• tests added / passed
• passes git diff upstream/master | flake8 --diff
• whatsnew entry

Added functionality in validate function of Rolling to ensure that window size is non-negative.

[codecov-io]

Current coverage is 84.32%

Merging #13441 into master will increase coverage by <.01%

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  Files           138        138          
  Lines         51068      51072     +4   
  Methods           0          0          
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+ Hits          43065      43069     +4   
  Misses         8003       8003          
  Partials          0          0          

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[jreback]

there is a another place where this is checked. in _Window (so need a test for that as well)

[jreback]

do this inside the elif

elif com.is_integer(window):
    if window < 0:
          raise ......
   ...

[priyankjain]

With this change, the test case
series.rolling(0, win_type='boxcar') fails,
should we change travis configuration file (requirements-3.5_OSX.build) to have scipy installation as part of the build or change the test case to handle the import error? @jreback

[jreback]

change the test case to skip if scipy is not installed

[jreback]

no, this is left off on purpose

[jreback]

make this a separate test (as skipping will cause the entire rest of the test to be skipped)

[jreback]

thanks!