Fix "ReferenceError: isDefined is not defined" error

It appears that the $LogProvider code was copied directly from the angular.js implementation, where 'isDefined' refers to the 'angular.isDefined' method. This fixes the angular-mocks file so that $LogProvider.debugEnabled uses angular.isDefined instead of isDefined.

(This PR can't be automatically merged, because there is no branch corresponding to the v1.2.0-rc.1 tag.)