API Enforce $allowed_actions in RequestHandler->checkAccessAction()

See discussion at https://groups.google.com/forum/?fromgroups#!topic/silverstripe-dev/Dodomh9QZjk

Fixes an access issue where all public methods on FormField were allowed,
and not checked for $allowed_actions. Before this patch you could e.g.
call FormField->Value() on the first field by using action_Value.

Removes the following assertion because it only worked due to RequestHandlingTest_AllowedControllerExtension
*not* having $allowed_extensions declared: "Actions on magic methods are only accessible if explicitly allowed on the controller."