[AC] REST client API tests - make JUnit tests

[joantune]

This is kinda of tricky because you would need an AC test instance to actually test the API (?) or do an elaborate Mock of it, that kinda would defeat the purpose (?). Anyway, I'm not sure what is the response, in case of edits, like:

• ACCategory - update method - what happens when one of the ids (project or category) is wrong. At least this would be useful to test