Fix smashing important state on GUILD_UPDATEs

GUILD_UPDATES are cool and special and of course they are partial.
Although this is logical, our type/models autoinitialize some fields by
default (which is actually fairly sane). However when this happens, we
smash these new blank mappings over the previously updated state.
Instead we should just ignore fields that don't come in GUILD_UPDATEs,
and save our state.

disco/state.py

@@ -193,7 +193,12 @@ class State(object):
             event.guild.sync()
 
     def on_guild_update(self, event):
-        self.guilds[event.guild.id].update(event.guild)
+        self.guilds[event.guild.id].update(event.guild, ignored=[
+            'channels',
+            'members',
+            'voice_states',
+            'presences'
+        ])
 
     def on_guild_delete(self, event):
         if event.id in self.guilds:

disco/types/base.py

@@ -327,8 +327,11 @@ class Model(six.with_metaclass(ModelMeta, AsyncChainable)):
             value = field.try_convert(raw, self.client)
             setattr(inst, field.dst_name, value)
 
-    def update(self, other):
+    def update(self, other, ignored=None):
         for name in six.iterkeys(self._fields):
+            if ignored and name in ignored:
+                continue
+
             if hasattr(other, name) and not getattr(other, name) is UNSET:
                 setattr(self, name, getattr(other, name))
 

disco/types/channel.py

@@ -149,8 +149,8 @@ class Channel(SlottedModel, Permissible):
         if not self.guild_id:
             return Permissions.ADMINISTRATOR
 
-        member = self.guild.members.get(user.id)
-        base = self.guild.get_permissions(user)
+        member = self.guild.get_member(user)
+        base = self.guild.get_permissions(member)
 
         for ow in six.itervalues(self.overwrites):
             if ow.id != user.id and ow.id not in member.roles: