Prevent response object being set as response body

While fixing earlier I just did `!is_object($result)` instead of `!($result instanceof CakeResponse)` but that broke lot of tests, for eg. `ObjectTest::testRequestActionParamParseAndPass()`.

Apparently when using requestAction a CakeRequest instance gets set as response body. I find that pretty strange. The docblock for `CakeReponse::body()` is incorrect in that case as it says the `$content` argument should be a string.