Fix various implementation bugs with allowed mentions

discord/abc.py

@@ -838,6 +838,8 @@ class Messageable(metaclass=abc.ABCMeta):
                 mentions = state.mentions.merge(mentions).to_dict()
             else:
                 mentions = mentions.to_dict()
+        else:
+            mentions = state.mentions and state.mentions.to_dict()
 
         if file is not None and files is not None:
             raise InvalidArgument('cannot pass both file and files parameter to send()')

discord/client.py

@@ -149,7 +149,7 @@ class Client:
         A status to start your presence with upon logging on to Discord.
     activity: Optional[:class:`.BaseActivity`]
         An activity to start your presence with upon logging on to Discord.
-    mention: Optional[:class:`AllowedMentions`]
+    mentions: Optional[:class:`AllowedMentions`]
         Control how the client handles mentions by default on every message sent.
 
         .. versionadded:: 1.4

discord/webhook.py

@@ -781,13 +781,15 @@ class Webhook:
         if username:
             payload['username'] = username
 
+        previous_mentions = getattr(self._state, 'mentions', None)
+
         if mentions:
-            try:
-                mentions = self._state.mentions.merge(mentions).to_dict()
-            except AttributeError:
-                mentions = mentions.to_dict()
-            finally:
-                payload['allowed_mentions'] = mentions
+            if previous_mentions is not None:
+                payload['allowed_mentions'] = previous_mentions.merge(mentions).to_dict()
+            else:
+                payload['allowed_mentions'] = mentions.to_dict()
+        elif previous_mentions is not None:
+            payload['allowed_mentions'] = previous_mentions.to_dict()
 
         return self._adapter.execute_webhook(wait=wait, file=file, files=files, payload=payload)