docs: public Hop header(String headerName, String headerValue) {
  
  		Assert.hasText(headerName, "headerName must not be null or empty!");
  
  		if (this.headers == HttpHeaders.EMPTY) {
  
  			HttpHeaders newHeaders = new HttpHeaders();
  			newHeaders.add(headerName, headerValue);
  
  			return new Hop(this.rel, this.parameters, newHeaders);
  		}
  
  		this.headers.add(headerName, headerValue);
  		return this;
  	}