Call iterateObserversSafely if vars change between calls to observer.next

[alessbell]

Closes #10105.

This PR updates the condition inside of src/core/ObservableQuery.ts's reportResult to check whether an observable query's variables have changed in between calls to observer.next. This resolves the linked issue: when loading remains true after observer.refetch is called multiple times with different variables when the same data are returned.

Previously, this would result in the lastError || this.isDifferentFromLastResult(result) check returning false, and iterateObserversSafely would not execute with the new result containing loading: false.

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

[benjamn]

I think we can simplify this a bit, and avoid introducing a new public method to the ObservableQuery class, which is a public API. Comments below!

[benjamn]

Looks good to me, thanks @alessbell. Please drop a note in CHANGELOG.md (in a new section for 3.7.1) and merge away!