[5.6] Allow asserting an integer with assertSee()

[shadoWalker89]

When trying to assert a number (example: id, downloads count ...) is seen or not seen on the page with any of these methods

assertSee() assertDontSee() assertSeeText() assertDontSeeText()

a phpunit exception will be raised, because the haystack (response content) is a string and the needle (what i'm asserting to be seen) is an integer (not a string)

Check the phpunit source code for that
https://github.com/sebastianbergmann/phpunit/blob/c1b35f210334bf1695af4fe6a16c3a72ebbe5e3d/src/Framework/Assert.php#L196

For example this assertion

$response = $this->get("some url");

$response->assertSee($paper->id);

Will raise this exception

Argument #1 (No Value) of PHPUnit\Framework\Assert::assertContains() must be a string
 F:\projects\quick-conference\src\quick-conference\vendor\laravel\framework\src\Illuminate\Foundation\Testing\TestResponse.php:284

This PR tries to solve that by casting the $value to a string