Ensure that assertSame generates diff for arrays

[mhujer]

Fixes #2169.

With this patch, the arrays are diffed for assertSame() similarly to assertEquals():

<?php declare(strict_types = 1);

use PHPUnit\Framework\TestCase;

class ArrayDiffTest extends TestCase
{
	public function testEquality()
	{
		$this->assertEquals(
			[1, 2, 3, 4, 5, 6],
			[1, 2, 33, 4, 5, 6]
		);
	}

	public function testSame()
	{
		$this->assertSame(
			[1, 2, 3, 4, 5, 6],
			[1, 2, 33, 4, 5, 6]
		);
	}

1) ArrayDiffTest::testEquality
Failed asserting that two arrays are equal.
--- Expected
+++ Actual
@@ @@
 Array (
     0 => 1
     1 => 2
-    2 => 3
+    2 => 33


2) ArrayDiffTest::testSame
Failed asserting that two arrays are identical.
--- Expected
+++ Actual
@@ @@
 Array &0 (
     0 => 1
     1 => 2
-    2 => 3
+    2 => 33

I also tested it with PHPStorm and it fixes the diffing issue there as well:

Questions:

1. is this a correct approach?
2. Wouldn't is be better to merge it to 6.5 branch?

[mhujer]

Appveyor failure seems unrelated. It is caused by http://windows.php.net/downloads/releases/php-7.2.0-Win32-VC15-x86.zip not being available (7.2.1 is available, so probably the version number should be incremented in AppVeyor config).

EDIT: See #2977