code: public AsyncRestBuilder withHeader(String key, Object value) {
          this.headers.put(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).