Add a "fetchHasChanged" method

[idolize]

This allows use of toPromise() when constructing fetchResult objects manually inside a store without relying on the store's this.fetch().

Very important for unit testing.

[taion]

Is this only for testing? I wonder if we can add it to some sort of test support library instead, if that's the case.

[idolize]

Testing is the main use case most likely, but there could be other times someone would want to use a toPromise(). For now a test library would be useful.