private void verifyAbsenceOfHeaders(MockRestResponseChannel restResponseChannel, String... headers) {
    for (String header : headers) {
      Assert.assertNull("[" + header + "] should not have been present in the response",
          restResponseChannel.getHeader(header));
    }
  }
Verifies that there are no values for all headers in {@code headers}.
@param restResponseChannel the {@link MockRestResponseChannel} over which response has been received.
@param headers the headers that must have no values.
ambry-frontend/src/test/java/com/github/ambry/frontend/AmbrySecurityServiceTest.java