removes 'const' from imap begin/end
removes 'const' from imap begin/end

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