[7.x] Add ability to check for multiple request headers when using HTTP Client

[mattkingshott]

This PR adds a hasHeaders() method to Illuminate/Http/Client/Request.php, and may be used to more succinctly check if an array of headers is present when inspecting requests. Under the hood, the hasHeaders method simply defers to the already present hasHeader method.

Before:

Http:withHeaders([
    'X-Test-Header' => 'foo',
    'X-Test-ArrayHeader' => ['bar', 'baz'],
]);

Http:assertSent(function ($request) {
    return $request->hasHeader('X-Test-Header', 'foo') &&
           $request->hasHeader('X-Test-ArrayHeader', ['bar', 'baz']);
});

After

Http:withHeaders($headers = [
    'X-Test-Header' => 'foo',
    'X-Test-ArrayHeader' => ['bar', 'baz'],
]);

Http:assertSent(function ($request) {
    return $request->hasHeaders($headers);
});

// or with PHP 7.4 arrow functions...

Http:assertSent(fn ($request) => $request->hasHeaders($headers));