Simplified check for digest in progress

Angular internally makes the check for digest in progress in this way rather than checking the string '$digest'.

https://github.com/angular/angular.js/blob/65ec0ab70a506b4b52fadecf444a7146d8d425e8/src/ng/http.js#L953

This check is also less likely to fail since a change to the string value ('$digest' to '$$digest' for instance) would break the check.