add support for allowed_mentions with message edit

discord/message.py

@@ -800,6 +800,10 @@ class Message:
             If provided, the number of seconds to wait in the background
             before deleting the message we just edited. If the deletion fails,
             then it is silently ignored.
+        allowed_mentions: Optional[:class:`~discord.AllowedMentions`]
+            Controls the mentions being processed in this message.
+
+            .. versionadded:: 1.4
 
         Raises
         -------
@@ -837,6 +841,18 @@ class Message:
 
         delete_after = fields.pop('delete_after', None)
 
+        try:
+            allowed_mentions = fields.pop('allowed_mentions')
+        except KeyError:
+            pass
+        else:
+            if allowed_mentions is not None:
+                if self._state.allowed_mentions is not None:
+                    allowed_mentions = self._state.allowed_mentions.merge(allowed_mentions).to_dict()
+                else:
+                    allowed_mentions = allowed_mentions.to_dict()
+                fields['allowed_mentions'] = allowed_mentions
+
         if fields:
             data = await self._state.http.edit_message(self.channel.id, self.id, **fields)
             self._update(data)