@Override
    public void filter(final ContainerRequestContext requestContext, final ContainerResponseContext responseContext) {
        responseContext.getHeaders().add(HeaderUtils.TIMESTAMP_HEADER, System.currentTimeMillis());
    }
@Test
    void testFilter() {
        final ContainerRequestContext        requestContext  = mock(ContainerRequestContext.class);
        final ContainerResponseContext       responseContext = mock(ContainerResponseContext.class);

        final MultivaluedMap<String, Object> headers         = HeaderUtils.createOutbound();

        when(responseContext.getHeaders()).thenReturn(headers);

        new TimestampResponseFilter().filter(requestContext, responseContext);

        assertTrue(headers.containsKey(org.whispersystems.textsecuregcm.util.HeaderUtils.TIMESTAMP_HEADER));
    }