sling-mock: Support creating nested resources with ContentBuilder

        Resource nested = context.create().resource("/my/path", 
                ImmutableMap.<String,Object>of(
                        "prop1", "value1",
                        "child1", ImmutableMap.<String,Object>of(
                            "prop2","value2",
                            "child2", ImmutableMap.<String,Object>of(
                                "prop3", "value3")
                            )
                        ));