fix: increment index to detect associative array

[Branlute]

Hello,

For a patch which remove elements to non associative array, we need to reindex this array.
This line is good

json-diff/src/JsonPointer.php

Line 297 in f4e5117

$parent = array_values($parent);

But the condition for $isAssociative has an issue.

We need to increment $i to compare with $index, otherwise $i will always be equal to 0 for all iterations.

[outdooracorn]

We've just stumbled across this as well and came up with a similar fix. Would love to see this merged soon. Pinging @vearutop for visibility.

[outdooracorn]

I decided to create #67 which includes a test case and increments the counter in the body of the foreach loop as I believe that to be easier to read.