Deprecate string options in URL helpers

Fixes https://github.com/rails/rails/issues/16958

I pretty much implemented @bronzle's fix in the issue comments, so he should have credit too.
Maybe it would be better to write

`if options && (deprecated_string_options = options & DEPRECATED_STRING_OPTIONS)`

and we use the `deprecated_string_options` variable to print the correct message and to iterate on the `each` loop.

This way to deleting an option that doesn't existe there.