CamelGpgContext: Enclose email addresses in brackets.

The recipient list for encrypting can be specified by either key ID or
email address.  Enclose email addresses in brackets to ensure an exact
match, as per the gpg man page:

HOW TO SPECIFY A USER ID

       ...

       By exact match on an email address.
              This is indicated by enclosing the email address in the
              usual way with left and right angles.

         <heinrichh@uni-duesseldorf.de>

Without the brackets gpg uses a substring match, which risks selecting
the wrong recipient.