Retain old handling of malformed addresses in To and Cc lines

The behaviour of the TMail backend's 'to' and 'cc' methods
where there was a malformed To: or Cc: line was to return
nil, whereas Mail returns a version of the string anyway.
We'd have to change quite a lot of code to deal with an
extra possible class of returned objects, so it's simplest
for the moment to monkey-patch Mail::Message's 'to' and 'cc'
methods to restore the old behaviour.