HTTP: Response checks

Naive implementation of #2072
Not sure about this.

We depend on Unsafe which is in sun.misc too, but I'm pretty sure it's available on non OpenJDK/Hotspot JVMs such as J9 and Azul. However, I'm not sure about `sun.misc.BASE64Decoder` (could be, just don't know). There's a `com.ning.http.util.Base64` in AHC that we could use instead.