Deprecate only_path: false option on *_path route helper

Addresses #17294

@chancancode I believe this handles all the cases you mention in #17294. Please let me know.

1) `*_url` helpers should continue to support the only_path: true | false option without deprecation
2) `*_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)
3) `*_url(...., only_path: true)` should not require the host option to be set (globally or as an argument)