Fixes an issue with libc++ eof handling in rapidjson

The credit to this fix is due to @zittix. This closes issue #51.

This is due to a bug in the mac version of libc++ and should be fixed in
HEAD. See here for a related stackoverflow post:
http://stackoverflow.com/questions/14147667/clang-and-libc-istreampeek-does-not-set-eof-flag