BUG: Make nansum work with booleans.

This broke when function_base._nannop tried to fill a boolean array with
integer zeros, raising a 'safe_casting' error. It looks like nanargmax and
nanargmin would also break, and were probably incorrect for booleans in any
case. The fix is not to use fill values for boolean and integer dtypes.
Previously that was only done for the integer dtypes.
Merge pull request #2979 from charris/fix-nansum

BUG: Make nansum work with booleans.