@Test public void constant() {

  BaggageField constant = BaggageFields.constant("foo", "bar");
  assertThat(constant.getValue(context)).isEqualTo("bar");
  assertThat(constant.getValue(extracted)).isEqualTo("bar");
  BaggageField constantNull = BaggageFields.constant("foo", null);
  assertThat(constantNull.getValue(context)).isNull();
  assertThat(constantNull.getValue(extracted)).isNull();
}