[5.4] Add new diffAssoc() method to collection

[MohannadNaj]

This PR introduces a Collection static function for using array_diff_assocon collections.

Using this suggested function diffAssoc, we can compare collections with respect to both keys and values.

From the PHP documentation:

array_diff_assoc ( array $array1 , array $array2 )
Compares array1 against array2 and returns the difference. Unlike array_diff() the array keys are also used in the comparison.

Usage: diff and diffKeys and diffAssoc:

$c1 = collect(
['id' => 1,
'username' => 'john',
'is_active' => 1,
'prefer_sms' => 1
]);

$c2 = collect(
	['id' => 2,
	'username' => 'doe',
	'is_active' => 1,
	]);

$c1->diff($c2);
//  ['username' => 'john']
//  the 'id' difference is ignored because the value of the 'id' index in the first collection, which is '1', is present in the 'is_active' index in the second collection.

$c1->diffKeys($c2);
//  ['prefer_sms' => 1]

$c1->diffAssoc($c2);
//  ['id' => 1, 'username' => 'john' , 'prefer_sms' => 1]