code: public RestBuilder withHeader(String key, Object value) {
          delegate.withHeader(key, value);
          return this;
      }
docs: Adds an HTTP header to the request. Using a key twice will result
  in the last value being used for a given header.
  
  @param key the header name (see HttpHeaders.Names for standard names).
  @param value the header value (see HttpHeaders.Values for standard values).