ArrayReduction does not update PathObservers

[peterwmwong]

When items are added or removed from array being watched by ArrayReduction, all the PathObservers that watched items indexed AFTER the add/remove index are out of sync.

Let's look at an example...

var values = ['a','b','c'];
var calls = [];
var reduction = new ArrayReduction(
  values,
  '',
  function(sum,cur){
    sum.push(cur);
    return sum;
  },
  calls
);
calls.splice(0);

// Internal view of observers.path for each item by index:
// array index    observer.path
// -----------    -------------
// 0              0
// 1              1             <---- To be spliced
// 2              2

values.splice(1,1);
reduction.deliver();

// Internal view of observers.path for each item by index:
// array index    observer.path
// -----------    -------------
// 0              0
// 1              2             <---- INCORRECT PATH, SHOULD BE 1


// == EXPECTED ==
// > Calls to reduce function: ["a", "c"]
// === ACTUAL ===
// > Calls to reduce function: ["a", "c", "a", "c", undefined]
console.log('Calls to reduce function:', calls);

... Notice the difference in EXPECTED and ACTUAL output.
What appears to be happening is the PathObserver that was watching the [2] element (which is now the [1] element), incorrectly causes a another reduce() because it sees a change in value ('c' -> undefined).

I believe the problem is that ArrayReduction's handleSplice() does not loop over and update the paths in observers. https://github.com/rafaelw/ChangeSummary/blob/08cb2c69b5daba0cbd5e806820311a76ea279858/util/array_reduction.js#L56

[rafaelw]

Well done!