BUG: fix casting to a GeoSeries in to_geodataframe under new xarray

[ilan-gold]

Fixes #111

See pydata/xarray#10278 for further discussion

[ilan-gold]

@martinfleis I think the whole test suite would benefit from a pass through at some point. The geom_array fixture is used repeatedly as a numpy array and then cast to a GeometryArray, it seems, to make testing for equality easier when xarray didn't handle this extension array stuff natively. So perhaps once the minimum version is bumped to a version of xarray where this geometry array type is preserved on the roundtrip, you can make geom_array natively as a fixture an extension array.

[martinfleis]

Thanks for the fix but it was only partial. This would result in columns in a GeoDataFrame being a Series with a geometry dtype, hence no geopandas methods would work on them. We always need to cast to a GeoSeries. What this uncovered is actually a geopandas bug that does not allow us to cast a Series with a geometry dtype to a GeoSeries as geopandas expects this to be a GeoSeries and have .crs attribute. Will report upstream.

I have pushed a version of the fix that goes around while keeping the GeoSeries casting for any geometry column.

[martinfleis]

It was actually fixed on geopandas main the meantime.