Fixed incorrect decoding of mailbox name... $_GET vars are already decoded.

Fixed incorrect decoding of mailbox name... $_GET vars are already decoded.
There are probably a whole bunch of other ones laying about, but this one
is fairly big in respect that you can view a folder that has a + in the name
(ie test+folder), but you cannot read mail in that folder.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@4610 7612ce4b-ef26-0410-bec9-ea0150e637f0