Fixes to work with latest pandas master

Recent changes  (particularly [#3482](https://github.com/pydata/pandas/pull/3482)) in [pandas/master](https://github.com/pydata/pandas) broke a couple things in GeoPandas.

A workaround for forcing the constructor to return a `GeoSeries` object is no longer necessary, and in fact now leads to inifinite recursion.  A call to `numpy.putmask` was failing because `pandas.core.Series` no longer inherits from `numpy.ndarray`.  Also added a more specific test for the `fillna()` method.