Fixed g_mime_header_list_register_writer() to not double-free the hash table key

2014-07-18  Jeffrey Stedfast  <fejj@gnome.org>

	* gmime/gmime-header.c (g_mime_header_list_register_writer):
	g_hash_table_remove() will g_free() the key for us, so don't do it
	ourselves (will result in a double-free).