Reflection field hints can downgrade allowWrite

[philwebb]

A field hint customizer can downgrade allowWrite if true is called before false. Here's a failing test:

	@Test
	void registerFieldWithCustomizersCannotDowngradeWrite() {
		Field field = ReflectionUtils.findField(TestType.class, "field");
		assertThat(field).isNotNull();
		this.reflectionHints.registerField(field, fieldHint -> fieldHint.allowWrite(true));
		this.reflectionHints.registerField(field, fieldHint -> fieldHint.allowWrite(false));
		assertTestTypeFieldHint(fieldHint -> {
			assertThat(fieldHint.getName()).isEqualTo("field");
			assertThat(fieldHint.isAllowWrite()).isTrue();
		});
	}