code: public HttpResponse withHeader(NottableString name, NottableString... values) {
          this.headers.withEntry(header(name, values));
          return this;
      }
docs: Add a header to return as a Header object, if a header with
  the same name already exists this will NOT be modified but
  two headers will exist
  
  @param name   the header name as a NottableString
  @param values the header values which can be a varags of NottableStrings