None of the usual parsing and normalization of email recipients was done
when forwarding tickets or transactions.  Use RT::EmailParser's
ParseEmailAddress method to convert to Email::Address objects
(effectively a split), then reformat and join back together.  This
ensures domain-less usernames are supported in forwards just like
elsewhere.  Importantly, this change also ensures the transaction
recording the forward action has a Data column which is guaranteed
parseable.

The forwarding tests previously tested a domain-less user in the same
field as a full email address, which is not supported by RT's email
parsing and would likely never work as expected with a real MTA.  Switch
to testing domain-less users are parsed correctly when in their own
recipient field.

These bugs were uncovered by RT::Transaction's new parsing of $txn->Data
for generating history descriptions.