Checking that the context that is being rest for a disposable context is indeed a disposable context

[rlester]

Hey @3lvis I am seeing an odd issue when using a disposable context combined with Sync syncing a background context.

What's happening is I have a disposable context that never has save called on it. It creates a temporary object that is just used to create a request. When that request returns it kicks off Sync which creates a background context.

I have spend a ton of time trying to track it down but for some reason when the background context is saved it causes the newDisposableMainContextWillSave to be triggered even though the observer was never set on the background context. This causes the background context which data I actually want to be stored to be reset and lost.

I also tried to create a unit test to reproduce this with no success. The even stranger part is it doesn't happen every time.

My fix is simple it just checks if the context calling newDisposableMainContextWillSave is really a disposable context. If not don't reset it.

[3lvis]

Good catch @rlester !