Fixed logic bug when removing a non-existent header

2014-02-19  Jeffrey Stedfast  <fejj@gnome.org>

	* gmime/gmime-message.c (message_remove_header): Don't clear the
	mime_part header stream unless we actually remove a header.
	(message_write_to_stream): Only add nwritten to the total if we've
	written a MIME-Version header.