@Test
    void requireThatKeySetWorksAsExpected() {
        Headers headers = new Headers();
        assertEquals(Set.of(), headers.keySet());
        headers.add("foo", "bar");
        assertEquals(Set.of("foo"), headers.keySet());
        headers.add("bar", "baz");
        assertEquals(Set.of("foo", "bar"), headers.keySet());
    }
@author Simon Thoresen Hult
@author Einar M R Rosenvinge
application/src/test/java/com/yahoo/application/container/handler/HeadersTestCase.java