TST: Assign and index using .loc

Fix the tests to assign using the loc indexer instead of assigning
to the resulting view. Fixes some pandas warnings.
Use pd.notnull() not bool()

bool(np.nan) evaluates to True. notnull() should be more of what
we want here.
BUG: interiors should return Series not GeoSeries

Since interiors returns InteriorRingSequence, we can't really
have it in a GeoSeries here. So return as Series.
BUG: Fix unary_union for GeoDataFrame
Merge pull request #170 from jwass/four_fixes

Four small fixes