Fix PartialEmoji._as_reaction() for emojis the bot can't see

If the bot has access to the emoji, you can pass anything for its name
and it will work fine. The name is only required to be correct when
adding a reaction where you do not have access to the emoji,
all other cases ignore the name.

discord/emoji.py

@@ -86,7 +86,7 @@ class PartialEmoji(namedtuple('PartialEmoji', 'animated name id')):
     def _as_reaction(self):
         if self.id is None:
             return self.name
-        return ':%s:%s' % (self.name, self.id)
+        return '%s:%s' % (self.name, self.id)
 
     @property
     def url(self):