ENH: merge() method to return GeoDataFrame also if only right frame is a GeoDataFrame

[jorisvandenbossche]

Pandas currently only checks the "left" frame to determine the class of the resulting DataFrame. This means that the order of the arguments passed matters: pd.merge(df, gdf, ..) (or df.merge(gdf, ..)) returns a DataFrame while pd.merge(gdf, df, ...) (or gdf.merge(df, ..)) returns a GeoDataFrame. This can be surprising for users.

I opened an issue for this on the pandas side: pandas-dev/pandas#44054 (as it needs to be solved there), but opening an issue here as well for visibility.