[commands] Make when_mentioned always have both mention formats.

This will allow it to work in mobile clients which don't respect the
<@!id> format in case of nicknames.

discord/ext/commands/bot.py

@@ -41,10 +41,7 @@ from .formatter import HelpFormatter
 def when_mentioned(bot, msg):
     """A callable that implements a command prefix equivalent
     to being mentioned, e.g. ``@bot ``."""
-    guild = msg.guild
-    if guild is not None:
-        return '{0.me.mention} '.format(guild)
-    return '{0.user.mention} '.format(bot)
+    return [bot.user.mention, '<@!%s>' % bot.user.id]
 
 def when_mentioned_or(*prefixes):
     """A callable that implements when mentioned or other prefixes provided.
@@ -62,7 +59,7 @@ def when_mentioned_or(*prefixes):
     """
     def inner(bot, msg):
         r = list(prefixes)
-        r.append(when_mentioned(bot, msg))
+        r.extend(when_mentioned(bot, msg))
         return r
 
     return inner