core: fix calls to domish.Element.elements:

domish.Element.elements should be called with namespace and element name as 2 arguments,
but it has been confused in several places with the call to `domish.Element.addElement`
which is often done with a `(namespace, name)` tuple. Unfortunately calling with a tuple
is accepted and doesn't raise any error in `elements`, but this result in a wrong element
returned.

This patch fixes the erroneous calls.

sat/plugins/plugin_sec_aesgcm.py

@@ -167,7 +167,7 @@ class AESGCM(object):
             attachments.clear()
             del data["extra"][C.MESS_KEY_ATTACHMENTS]
 
-        body_elt = next(data["xml"].elements((C.NS_CLIENT, "body")))
+        body_elt = next(data["xml"].elements(C.NS_CLIENT, "body"))
 
         for attachment in attachments:
             body_elt.addContent(attachment["url"])