Added alternative spelling of osgb36 as osgb_1936 to match PRJ files.

Fixed a problem with creating an OS Grid transform from a well known text file, where the datum transformation would not be looked up properly.

in WKT format, the datum is D_OSGB_1936, which gets turned internally into osgb_1936. In proj4 format, the datum is OSGB36.

See http://spatialreference.org/ref/epsg/osgb-1936-british-national-grid/esriwkt/ and http://spatialreference.org/ref/epsg/osgb-1936-british-national-grid/proj4/

I've fixed this by creating another copy of the constant with the wkt name, but I don't know the codebase that well, so it may be better to fix this in another place.