Assert that instance of cherry picked

This is to replace the pull request
https://github.com/apache/wicket/pull/57

The commit 3744d5b was inspired by a non informativ test fail.
Like the assert
assertTrue(factory.getFieldValue(field, obj) instanceof ILazyInitProxy);
simply fails with no further information.
As org.hamcrest.CoreMatchers is already pulled into the classpath by junit it might be ok to transform the given assertTrue to:
assertThat(factory.getFieldValue(field, obj), instanceOf(ILazyInitProxy.class));

Now when the assertion fails the value of the first argument is printed
in the test output.