Fix for #3150

When credit card expiration fields are set to display as a menu (the
current default), they ignore all theme tag options (most notably the
“required” option). This change sets those fields to required by
default, and allows a smaller range of applicable input attributes.

Also removed a few instances of duplicate “required” classes in other
API tags.
Renamed allowed array

$allowed > $allowedKeys
Removed debug code

A couple of calls to shopp_debug that aren’t needed
Fixes re: @jonathandavis

A better approach.
Merge branch 'cc-expire-fields-change' of https://github.com/crunnells/shopp into crunnells/pr/3151
Merge branch 'crunnells/pr/3151' into 1.3.x