groupby does end-of-group check in ++
groupby does end-of-group check in ++

This way it's only done once, and it avoids constness issues arising
elsewhere when it was in operator!=

jkhoogland/cppitertools,tempbottle/cppitertools,ryanhaining/cppitertools,tempbottle/cppitertools,ryanhaining/cppitertools,tempbottle/cppitertools,ryanhaining/cppitertools,jkhoogland/cppitertools,jkhoogland/cppitertools