fix issue 103: https://github.com/lisong/code-push-server/issues/103

[jeoy]

according https://github.com/Microsoft/code-push/blob/master/sdk/script/management-sdk.ts#L337

   if (expectResponseBody && !body) {
        reject({ message: "Could not parse response: " + res.text, statusCode: res.status });
   }

if code-push successfully release a new packages with sending '' as response text, we always encounter this error 'Could not parse response:'.

Instead we should send a JSON string as response text, like '{"msg": "succeed"}'.