When creating customField via the API, can't set option_group_id

If you create customFields programmatically, you might want to use the same option group over and over again.

For instance, if you're creating custom fields for Writing Ability, Teaching Ability, and Spanish Language Ability, you might create an option group called Skill Level (expert, intermediate, novice) and use it for all three.

Currently the create API disregards the option_group_id parameter. The only option is to specify option values via the option_values parameter, in which case you'd end up with three nearly identical option groups.