Enable Upload test once #354 is fixed

[denis-anisimov]

The main issue is #354 It causes https://github.com/vaadin/vaadin-upload-flow/issues/84

There an IT test for the https://github.com/vaadin/vaadin-upload-flow/issues/84. Currently it's disabled because the issue is not fixed.

Here is the test method: https://github.com/vaadin/spring/blob/master/vaadin-spring-tests/test-spring-common/src/test/java/com/vaadin/flow/spring/test/UploadIT.java#L38

It should be enabled once #354 is fixed.