code: public TokenRequest setScopes(Collection<String> scopes) {
      this.scopes = scopes == null ? null : Joiner.on(' ').join(scopes);
      return this;
    }
docs: Sets the list of scopes (as specified in <a
  href="http://tools.ietf.org/html/rfc6749#section-3.3">Access Token Scope</a>) or {@code null}
  for none.
  
  <p>
  Overriding is only supported for the purpose of calling the super implementation and changing
  the return type, but nothing else.
  </p>
  
  @param scopes collection of scopes to be joined by a space separator (or a single value
  containing multiple space-separated scopes)
  @since 1.15