BUG: Check 'method' in GeoDataFrame __finalize__

This will pull metadata from the correct place when the finalize
operation is from a merge.

This partly addresses Issue #118, but it's a bug in Pandas that hardcodes `DataFrame` as the result of a merge.

The new test is skipped here until Pandas [PR#7737](https://github.com/pydata/pandas/pull/7737) is merged.
Not yet ready to merge.