Improves Java REST tests: unique Strings, negative assertions

[ignasi35]

This PR introduces multiple changes:

1. each test case uses a unique title and body on the data it creates. This is necessary because this is a stateful tests and assertions were being mixed up
2. assertions are more strict. Previous implementation would treat the response as a plain String and assert that contained the body substring. This assertion is wrong since the Play failure page (an HTML indicating the requested URI is missing) can be converted to a String and it also contains the substring body.
3. Adds a test case to ensure GET /v1/posts/ works. Equivalent to Small fixes for Play REST samples #48 but for the java sample.

This PR is currently not passing all the tests (tested in 2.8.0-M3 and 2.7.3). The reason is a test expecting GET /v1/posts/ (the requests includes a trailing slash) fails.

This is related to playframework/playframework#8850, #47 and #48

[ignasi35]

After discussing with @marcospereira, the behavior of Play is correct and the failing test here is wrong: the test expects GET /v1/posts/ to return a list of values but that's the wrong expectation.

Note that in play-scala-rest-example there's a similar test which does the exact same assertion and passes the test. The reason is that play-scala-rest-example sneakily includes support for trailing slashes but that's not a vanilla Play feature.

[marcospereira]

Oh... this is API is so nice. /s

[marcospereira]

👍