Inconsistent patch generation when adding `undefined` elements to an array

[eriksunsol]

This is a continuation of dicussion here: https://github.com/Palindrom/JSONPatcherProxy/pull/45#discussion_r311897775

Steps to reproduce

See failing test here: 7cf32fd09593fef2751abc51114267e4a14f3897

The test extends an array with 5 elements to 9 elements with undefined or null values.

Expected result

Four add patches to add null elements.

Actual result

One add patch and one replace patch.

[warpech]

I decided to implement the solution in the already opened PR https://github.com/Palindrom/JSONPatcherProxy/pull/45, because I was afraid of creating a conflict that's troublesome to merge.

[warpech]

This issue is partially solved in #45, but there are even more edge cases:

There are 3 kinds of arrays to test:

• array of values other than undefined
• array of undefined
• of empty slots (https://stackoverflow.com/questions/40752434/how-to-tell-between-undefined-array-elements-and-empty-slots/40752485)

There are many kinds of array operations to test:

• array.shift()
• array.pop()
• array.unshift()
• array.push()
• array.splice()
• array.length