Use channel_id instead of channel to get id

Previously, this line caused a crash when sending responses in a guild
that uses the applications.commands scope. In this case,
SlashContext.channel is None and SlashContext.channel_id has the
channel id.

This appears to be a holdover from pre-1.0.9 when SlashContext.channel
represented either a channel or a channel id.

Author: MichaelJHarvey

discord_slash/context.py

@@ -184,7 +184,7 @@ async def send(self,
         resp = await self._http.post(base, self.interaction_id, self.__token, files=files)
         smsg = model.SlashMessage(state=self.bot._connection,
                                   data=resp,
-                                  channel=self.channel or discord.Object(id=self.channel),
+                                  channel=self.channel or discord.Object(id=self.channel_id),
                                   _http=self._http,
                                   interaction_token=self.__token)
         if delete_after: