Prevent unhandled action from returning new state reference

[aduth]

This pull request seeks to (a) resolve an issue where unhandled actions were inadvertently creating a new state reference and (b) introduce a new test to guard against such cases in the future. A Redux reducer should ideally never return a new reference if no changes have occurred, and especially if the action is never handled. It was found that combineUndoableReducers was testing the incorrect property when deciding whether to return the same state reference and therefore returned a new reference every time the reducer was invoked. This has been resolved here, and a test case added to protect against future regressions.

Testing instructions:

There should be no behavioral changes in the editor.

Ensure tests pass:

npm test

cc @youknowriad , this was cause of issues described at #1077 (comment) (state did in-fact change, or at least the reference did).

[aduth]

I added fe3cc68, which is not entirely related, but a small fix after observing that a new post incorrectly has undo history.

[aduth]

After #1063, it appears the changes from fe3cc68 are no longer necessary so have been dropped in rebase.

[aduth]

@youknowriad Appears redux-optimist returns a new object reference on every dispatch, causing the tests to fail. Reported at ForbesLindesay/redux-optimist#22 .

[aduth]

I'm going to rebase to drop the commit adding the base store change protection test, and move forward with the fix to undoableReducer because I'm encountering issues in another pull request.

[aduth]

Original commit for store change protection test (for future reference): d0158e0