Description

This patch:

• makes radio_button respect the :index option
• makes label respect the :index option
• adds test coverage for all form_helper :index behavior
• adds a :radio_button_value key to label so that it can generate consistent :for attributes for radio_button tags (previously developers had to generate their own :for attributes for labels of radio buttons)
• adds documentation for the radio_button index and auto_index behavior
• adds documentation for the new label :radio_button_value key

All actionpack tests pass, but I did not run the active_record_integration tests.

This is my first patch here, and I know that the :radio_button_value is a bit of a stretch. If you want me to rename that key, or remove it from the patch I can.

Also, if there are any stylistic issues I'd be glad to fix them.