GTiff/internal libgeotiff: fix handling of Mercator_2SP when reading/writing it (without associated EPSG code, patch by Trent Piepho); resync with libgeotiff when reading geotiff information with only a PCS corresponding to a Mercator_2SP projection (#5791)

git-svn-id: https://svn.osgeo.org/gdal/trunk@28237 f0d54148-0727-0410-94bb-9a71ac55c965