Missing per-request verification

New _getPlaybackRequest(..) needed to be like _getPlaybackResponse(..)

Playback headers, and playback body (normally GETs have no body) need to match what was previously recorded. If they don't servirtium should force a failure in its response, and indicate the root cause in `getLastError()`

See https://github.com/servirtium/README/blob/master/starting-a-new-impl-step-10.md