code: @Override
      public StreamResponse header(String name, String... values) {
          assertArgumentNotNull("name", name);
          assertArgumentNotNull("values", values);
          assertDefinedState("header");
          if (headerMap.containsKey(name)) {
              throw new IllegalStateException("Already exists the header: name=" + name + " existing=" + headerMap);
          }
          headerMap.put(name, values);
          return this;
      }
docs: ======