fix(core): Ruby 2.7 compatibility
fix(core): Ruby 2.7 compatibility

* Update Cloud::Config#method_missing to be compatible with
  an upcoming change in Symbol#to_s returning a frozen string.

pr: #4109
dazuma/google-cloud-ruby,googleapis/google-cloud-ruby,googleapis/google-cloud-ruby,googleapis/google-cloud-ruby,dazuma/google-cloud-ruby,googleapis/google-cloud-ruby,dazuma/google-cloud-ruby,dazuma/google-cloud-ruby,googleapis/google-cloud-ruby,dazuma/google-cloud-ruby,googleapis/google-cloud-ruby,dazuma/google-cloud-ruby