3 Small Fixes

There are 3 small commits here. For whatever reason, the 3.2 builds in Travis are all hanging on the matplotlib install right now but are not failing...

* 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.