Fix to BUG GH11698. added default value of mask
closes #11698

Please review.

minor change, ping when green.
 modified test case. 
This branch has some conflicts . Please check.
 u need to fix the conflict 
 @jreback  There is a conflict in whatsnew file. When I am adding the bug fix message and trying to merge with the latest code , it always shows me conflicts. I merged my whatsnew file with the latest code but there is still a conflict. Please help.
 lots of help on this: http://githowto.com/resolving_conflicts

you basically edit the conflicted file, picking the correct parts out. then add the files, rebase, and push
 merged via https://github.com/pydata/pandas/commit/8ec8487a4a8362ac94630388f217f273b3ad1525

thanks!
 Thanks 
