Adds test with const deref of product
Adds test with const deref of product

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