Update for GF 1.7 compat and functioanlity change

Doesn't make the UI look pretty with GF 1.7, but makes it functional. Also updated the way this works so that empty values from the saved $lead object do not overwrite populated values in the $_POST. This prevents issues on multi-page forms where changes made to the $_POST via the form are overwritten on subsequent page loads by the empty values in the save $lead.