Add the timestamp using the add method.

service/src/test/java/org/whispersystems/textsecuregcm/filters/TimestampResponseFilterTest.java

@@ -0,0 +1,31 @@
+package org.whispersystems.textsecuregcm.filters;
+
+import org.glassfish.jersey.message.internal.HeaderUtils;
+import org.junit.Test;
+import org.whispersystems.textsecuregcm.util.TimestampHeaderUtil;
+
+import javax.ws.rs.container.ContainerRequestContext;
+import javax.ws.rs.container.ContainerResponseContext;
+import javax.ws.rs.core.MultivaluedHashMap;
+import javax.ws.rs.core.MultivaluedMap;
+
+import static org.junit.Assert.*;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+public class TimestampResponseFilterTest {
+
+    @Test
+    public 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(TimestampHeaderUtil.TIMESTAMP_HEADER));
+    }
+}