code: public <R> R getHeader(String name, R defaultValue) {
          if (headers().contains(name)) {
              return getHeader(name);
          } else {
              return defaultValue;
          }
      }
docs: Allow getting headers in a generified way and return defaultValue if the key does not exist.
  
  @param name The key to get
  @param defaultValue value returned when the key does not exist
  @param <R> The type of the return
  @return The found object