Fix S/MIME sending and Outlook2007 reception

	* libbalsa/rfc3156.c (libbalsa_message_body_protection):
	x-pkcs7-mime is synonymous with pkcs7-mime;
	(libbalsa_encrypt_mime_object): use pkcs7 instead of leaking it.
	* src/balsa-message.c (libbalsa_msg_try_decrypt):
	x-pkcs7-mime is synonymous with pkcs7-mime.