code: public final void fromHttpHeaders(HttpHeaders headers) {
      try {
        ParseHeaderState state = new ParseHeaderState(this, null);
        serializeHeaders(
            headers, null, null, null, new HeaderParsingFakeLevelHttpRequest(this, state));
        state.finish();
      } catch (IOException ex) {
        // Should never occur as we are dealing with a FakeLowLevelHttpRequest
        throw Throwables.propagate(ex);
      }
    }
docs: Puts all headers of the {@link HttpHeaders} object into this {@link HttpHeaders} object.
  
  @param headers {@link HttpHeaders} from where the headers are taken
  @since 1.10