Merge branch 'form-multi-attrib' into 'master'

XEP-0004: add "forms" as a multi-attrib plugin for Form

See merge request poezio/slixmpp!147

slixmpp/plugins/xep_0004/stanza/form.py

@@ -19,6 +19,7 @@ class Form(ElementBase):
     namespace = 'jabber:x:data'
     name = 'x'
     plugin_attrib = 'form'
+    plugin_multi_attrib = 'forms'
     interfaces = OrderedSet(('instructions', 'reported', 'title', 'type', 'items', 'values'))
     sub_interfaces = {'title'}
     form_types = {'cancel', 'form', 'result', 'submit'}