code: public HttpHeaders setAll(HttpHeaders headers) {
          checkNotNull(headers, "headers");
  
          if (headers.isEmpty()) {
              return this;
          }
  
          for (Entry<String, String> entry : headers) {
              set(entry.getKey(), entry.getValue());
          }
          return this;
      }
docs: Retains all current headers but calls {@link #set(String, Object)} for each entry in {@code headers}
  
  @param headers The headers used to {@link #set(String, Object)} values in this instance
  @return {@code this}