check handler type on construction

[DQNEO]

Background

handler option must be an callable, but there is no type check

Current behavior

So Fatal Error occurs on request time.

$client = new GuzzleHttp\Client(['handler' => 'x']);
$res = $client->request('get', 'https://example.com/');

PHP Fatal error:  Uncaught Error: Call to undefined function x() in /Users/DQNEO/src/github.com/DQNEO/guzzle/src/Client.php:275
Stack trace:
#0 /Users/DQNEO/src/github.com/DQNEO/guzzle/src/Client.php(123): GuzzleHttp\Client->transfer(Object(GuzzleHttp\Psr7\Request), Array)
#1 /Users/DQNEO/src/github.com/DQNEO/guzzle/src/Client.php(129): GuzzleHttp\Client->requestAsync('get', Object(GuzzleHttp\Psr7\Uri), Array)
#2 /Users/DQNEO/src/github.com/DQNEO/guzzle/,.php(9): GuzzleHttp\Client->request('get', 'https://httpbin...')
#3 {main}
  thrown in /Users/DQNEO/src/github.com/DQNEO/guzzle/src/Client.php on line 275

New behavior

by this PR, users can see a exception on construction , which is more easier to find the cause.

PHP Fatal error:  Uncaught InvalidArgumentException: handler must be a callable in /Users/DQNEO/src/github.com/DQNEO/guzzle/src/Client.php:66
Stack trace:
#0 /Users/DQNEO/src/github.com/DQNEO/guzzle/,.php(7): GuzzleHttp\Client->__construct(Array)
#1 {main}
  thrown in /Users/DQNEO/src/github.com/DQNEO/guzzle/src/Client.php on line 66

[mtdowling]

The PR looks good. This if statement could be simplified though:

if (!isset($config['handler'])) {
    $config['handler'] = HandlerStack::create();
} elseif (!is_callable($config['handler'])) {
    throw new \InvalidArgumentException('handler must be a callable');
}

[DQNEO]

I fixed it!

[sagikazarmark]

Thanks for your contribution