code: @Override
      public boolean containsHeader(String name) {
          return (headers != null && headers.get(name) != null && !headers.get(name).isEmpty());
      }
docs: Returns a boolean indicating whether the named response header
  has already been set.
  
  @param name the header name
  @return {@code true} if the named response header
  has already been set; {@code false} otherwise