@Test
void testPutReplace() {

    MultiMap<String> mm = new MultiMap<>();
    String key = "formats";
    Object ret;
    ret = mm.put(key, "gzip");
    assertMapSize(mm, 1);
    assertValues(mm, key, "gzip");
    assertNull(ret);
    Object orig = mm.get(key);
    // Now replace it
    ret = mm.put(key, "jar");
    assertMapSize(mm, 1);
    assertValues(mm, key, "jar");
    assertEquals(orig, ret);
}