Four small fixes

Replaces #163 - now with 4 small commits. Tests pass now that 3.2 is no longer supported.

* Fixes a few tests to assign to subset of rows using `.loc[...]` instead of trying to assign to a view or copy
* Use `if pd.notnull(item)` instead of `if item` when generating the rtree stream input. `if item(np.nan)` returns True!!
* The `interiors` property returns a `Series` instead of a `GeoSeries` as it's not really a proper `geometry` and caused issues when the index was being generated.
* Fix `unary_union` to work on `GeoDataFrame`.