test($rootScope): add assertion to test ensuring that NaN -> NaN does not throw
test($rootScope): add assertion to test ensuring that NaN -> NaN does not throw

https://github.com/angular/angular.js/commit/aac961be85af2d4d2223c3a433fb56abf6f76c0b implements a
fix for NaN values causing $watchCollection to throw an infdig error. This change updates the test
by adding an assertion which explains what is actually being tested a bit better, and may also
provide better information in the event that the test ever fails.

Closes #6758

ggershoni/angular.js,ggershoni/angular.js,ggershoni/angular.js,ggershoni/angular.js