@Override protected void propagationField(String keyName, String value) {

    if (keyName == null) throw new NullPointerException("keyName == null");
    if (value == null) throw new NullPointerException("value == null");
    Key<String> key = nameToKey.get(keyName);
    if (key == null) {
      assert false : "We currently don't support setting headers except propagation fields";
      return;
    }
    headers.removeAll(key);
    headers.put(key, value);
  }
@Test void propagationField_replace() {
    headers.put(b3Key, "0");

    request.propagationField("b3", "1");

    assertThat(request.headers.get(b3Key)).isEqualTo("1");
  }