@Context object now injected when unit testing resteasy

I'm trying to register a @Provider which would inject Locale object into the @Context so that I can reuse it in my REST resources.

Following the article http://ift.tt/1MmqMdg I managed to get the provider running

@Provider
public class LocaleProvider implements ContainerRequestFilter {

    public static final String DEFAULT_LANGUAGE_CODE_VALUE = "en-US";

    @Context
    Dispatcher dispatcher;

    @Override
    public void filter(ContainerRequestContext containerRequestContext) throws IOException {
        List<Locale> acceptableLanguages = containerRequestContext.getAcceptableLanguages();
        if (acceptableLanguages == null || acceptableLanguages.isEmpty()) {
(A)         dispatcher.getDefaultContextObjects().put(Locale.class, new Locale(DEFAULT_LANGUAGE_CODE_VALUE));
            return;
        }

        dispatcher.getDefaultContextObjects().put(Locale.class, acceptableLanguages.get(0));
    }


}

And use it like this

@Path("/")
public class Resource {

    @GET
    public String get(@Context Locale locale) {
        if (locale == null) {
            return "null";
        }

        return locale.getLanguage();
    }
}

The code so far works great when deployed to Tomcat. But I run into an issue when I try to unit test it. I have

public class LocaleProviderTest {
    private Dispatcher dispatcher = MockDispatcherFactory.createDispatcher();

    {
        dispatcher.getRegistry().addResourceFactory(new POJOResourceFactory(Resource.class));
    }

    @Test
    public void shouldHaveDefaultLocaleWhenNoAcceptLanguageHeader() throws URISyntaxException {
        dispatcher.getProviderFactory().registerProvider(LocaleProvider.class);

        MockHttpRequest request = MockHttpRequest.get("/");
        MockHttpResponse response = new MockHttpResponse();
        dispatcher.invoke(request, response);

        assertEquals("en", response.getContentAsString());
    }
}

The Resource returns null even though the LocaleProvider executes the line (A) if I debug it.

Any idea why the @Context injection does not work when unit testing?