Typehint on \Github\Api\Repo->dispatch is wrong

Github wants an object. We're passing an array. I added `$clientPayload = (object) $clientPayload;` as the first line of this method to verify. Typehinting `object $clientPayload` is probably better, but I'm not sure we can really change it without a BC break? Then again, if anyone was attempting to use this, they would have flagged this issue already.

PR incoming.