code: public T header(Resource resource, Charset charset) {
          try {
              getMessageContentBuilder().getHeaderData().add(FileUtils.readToString(resource, charset));
          } catch (IOException e) {
              throw new CitrusRuntimeException("Failed to read header resource", e);
          }
  
          return self;
      }
docs: Expect this message header data in received message from file resource. Message header data is used in
  SOAP messages as XML fragment for instance.
  @param resource
  @param charset
  @return