Add integration tests

[philwebb]

Unless I'm missed them, it looks like there currently aren't any integration tests for Spring Data Rest. Whilst looking at #1959 I couldn't find anywhere to add a test that replicated the problem before attempting to fix it. I needed to run the Spring Boot integration test instead, which adds some time to the process.

[odrotbohm]

There's an extensive set using all supported Spring Data stores in spring-data-rest-tests. Running the build with the it profile enabled runs those.

[philwebb]

I missed those. I tried some things that I expected to fail but I wasn't aware of the it profile so they didn't. Perhaps this issue could be a documentation update to the CONTRIBUTING instructions?