Not 100% sure why order is in this way (I think its order a-z)... however I have always avoided relying on key order(learnt the hard way in Perl which doesn't enforce key order in hashes). What you could do here is three assertJsonFragments on each of the keys but TBH I normally just decode the json and store in a variable then use phpunit assertions
e.g. $this->assertEquals('UTC',$response['created_at']['timezone']);
Remember the client consuming your api won't care about the order of the keys (it certainly shouldnt)