Don't assume that any shared fields are optional in Client.edit_channel.

Thanks Jake and night.

discord/client.py

@@ -1522,8 +1522,10 @@ class Client:
             Editing the channel failed.
         """
 
-        if 'name' not in options:
-            options['name'] = channel.name
+        keys = ('name', 'topic', 'position')
+        for key in keys:
+            if key not in options:
+                options[key] = getattr(channel, key)
 
         yield from self.http.edit_channel(channel.id, **options)
 

discord/http.py

@@ -339,7 +339,7 @@ class HTTPClient:
     def edit_channel(self, channel_id, **options):
         url = '{0.CHANNELS}/{1}'.format(self, channel_id)
 
-        valid_keys = ('name', 'topic', 'bitrate', 'user_limit')
+        valid_keys = ('name', 'topic', 'bitrate', 'user_limit', 'position')
         payload = {
             k: v for k, v in options.items() if k in valid_keys
         }