rss: watch handle case during rename

rss: watch handle case during rename

We allow rss handles to be of any case on creation, even though matching
is case-insentivie. However, when renaming an rss using 'rss change
handle' and only changing the case, two things prevented this from
working correctly:
* since the new downcased handle was equal to the old downcased handle,
  the bot would prevent the renaming due to the existence of the new
  handle
* the new handle was forcefully downcased, preventing the user
  from renaming handle 'case' to 'CaSe'.

Fix by checking for this case explicitly, and handling it separately.