[boost] [Iterator] Fixing the default reference type of transform_iterator (#5127)

The default reference type of transform_iterator should be boost::result_of<const UnaryFunction(Iterator::reference)> rather than boost::result_of<UnaryFunction(Iterator::reference)> since the dereference operator of transform_iterator is a const member function. Ticket #5127 describes this bug https://svn.boost.org/trac/boost/ticket/5127 and the ticket has a patch. Jeff, could you apply the patch, if it looks OK? Regards, Michel