Fix mutation after push

[cherifGsoul]

Fix the item mutation with array mutation methods like push:

const myList = new ObservableArray([0,1]);
myList.push("I am going to hide some changes.");
		
canReflect.onPatches(myList, function (patches) {
    console.log(patches[0].insert[0]);   // -> 'Patched after push'
});
	
myList[1] = "Patched after push";

Closes #82

[phillipskevin]

@cherifGsoul can you explain why there are more assertions now?

[phillipskevin]

Also, do you know why this property is not initialized? It looks like it should be here:

can-observable-array/src/proxy-array.js

Line 258 in 1c50a7d

preventSideEffects: 0

.