Deprecate only_path: false option on *_path route helper

@chancancode I believe this handles all the cases you mention [here](https://github.com/rails/rails/issues/17294). Please let me know.

*_url helpers should continue to support the only_path: true | false option without deprecation
*_path helpers should continue to support the only_path: true | false option with a deprecation warning that (*_path(..., only_path: false) would not work on Rails 5, use *_url instead)
*_url(...., only_path: true) should not require the host option to be set (globally or as an argument)