code: public RestClientOptions putGlobalHeaders(MultiMap headers) {
          for (Map.Entry<String, String> header : headers) {
              globalHeaders.add(header.getKey(), header.getValue());
          }
  
          return this;
      }
docs: Set global headers which will be appended to every HTTP request.
  The headers defined per request will override this headers.
  
  @param headers The headers to add
  @return a reference to this so multiple method calls can be chained together