Follow up to #17131. Addresses #17894.

BaseService#root_url is documented as an Addressable::URI, but in BaseService#root_url= it is used as if it is a String. This results in the following error:

google-apis-core-0.13.0/lib/google/apis/core/base_service.rb:131:in `root_url=': undefined method `include?' for an instance of Addressable::URI (NoMethodError)

          if url_or_template.include? ENDPOINT_SUBSTITUTION
                            ^^^^^^^^^

The tests seem to indicate that root_url was always allowed to be a String and I don't see any code prior to #17131 which required it to be an Addressable::URI. Therefore I am proposing updating the documentation to indicate that it can be either.