Support guzzlehttp/guzzle:^8.0.2

[juukie]

Having a new major version of guzzlehttp/guzzle ^8.0.2, it would be nice if this package gets compatible.

As far as I could see:

Header values passed to guzzle should be of type string. For example:

// Before
$headers = [
    'Resource-ID' => 123,
];

// After
$headers = [
    'Resource-ID' => '123',
];

Guzzle GuzzleHttp\Psr7\Request method isn't uppercased

When using new GuzzleHttp\Psr7\Request(..), the method isn't uppercased anymore. Resulting in one assert to fail in IncomingWebhookTest::testMakesWebhookFromGlobals.

Guzzle's RequestException constructor's signature changed.

// < 8
public function __construct(string $message, RequestInterface $request, ?ResponseInterface $response = null, ?\Throwable $previous = null, array $handlerContext = [])

// 8
public function __construct(string $message, RequestInterface $request, int $code = 0, ?\Throwable $previous = null)

There are three exceptions extending the RequestException:

• AuthenticationException
• RateLimitExceededException
• ValidationErrorException

I'd love to contribute to making it compatible, but don't know what steps would be best.