BUG: test case showing why assigning to dtype is unsafe
BUG: fix unsafe dtype changes in putmasking on series
     allow boolean indexing of series w/o changing the dtype
     with a list of the rhs if we can preserver the dtype
     of the input, if we can't upcast, but can only do this
     in cases where we won't change the itemsize
Merge pull request #3292 from jreback/unsafe

BUG: fix unsafe dtype conversion in Series