fix($rootScope): handle cyclic references in scopes when creating error messages

Use the new private function `stringify` to convert scope values to strings,
since this can cope with cyclic references and other oddities.

Author: petebacondarwin

src/ng/rootScope.js

@@ -776,7 +776,7 @@ function $RootScopeProvider() {
                         logMsg = (isFunction(watch.exp))
                             ? 'fn: ' + (watch.exp.name || watch.exp.toString())
                             : watch.exp;
-                        logMsg += '; newVal: ' + toJson(value) + '; oldVal: ' + toJson(last);
+                        logMsg += '; newVal: ' + stringify(value) + '; oldVal: ' + stringify(last);
                         watchLog[logIdx].push(logMsg);
                       }
                     } else if (watch === lastDirtyWatch) {
@@ -810,7 +810,7 @@ function $RootScopeProvider() {
             throw $rootScopeMinErr('infdig',
                 '{0} $digest() iterations reached. Aborting!\n' +
                 'Watchers fired in the last 5 iterations: {1}',
-                TTL, toJson(watchLog));
+                TTL, watchLog);
           }
 
         } while (dirty || asyncQueue.length);