code: public OkRequest<T> headers(final Map<String, String> headers) {
          if (!headers.isEmpty())
              for (Map.Entry<String, String> header : headers.entrySet())
                  header(header);
          return this;
      }
docs: Set all headers found in given map where the keys are the header names and
  the values are the header values
  
  @param headers
  @return this request