Improve api consistency on custom field creation

Api supports a key 'option_values' by always setting the 'magic' param
option_type to 1. v4 doesn't - but creating option values when creating
a custom field is desirable. This fixes so that the form still
'opts out' if it passes in '2' but otherwise we create the option values
if passed in.

Just a teensy bit less magic in the api layer & bao