dev/core#1551 regression fix

dev/core#1551 regression fix

Now that mailing_id is defined as field metadata the form main metadata fn is trying to handle it, However
the form is casting it to the wrong array format -https://github.com/civicrm/civicrm-core/blob/86f20cc17dde028977130f84feaf3b1d760ca57d/CRM/Contact/Form/Search.php#L649

We could fix  this & rip it out of Mailing_BAO_Query::where but there is still a gap  in  that
there is no pseudoconstant recorded for the field so the qill doesn't work & there is a gap in
pre-parsing for the form 'IN'

Hence this seems like a good approach to remove the regression in the immediate term