Fix deprecation warning in PHP7.4

Author: guywithnose

src/GuzzleAdapter.php

@@ -92,7 +92,7 @@ public function end(string $endHandle) : ResponseInterface
 
         $this->promises = [];
 
-        if (array_key_exists($endHandle, $this->exceptions)) {
+        if ($this->exceptions->offsetExists($endHandle)) {
             $exception = $this->exceptions[$endHandle];
             unset($this->exceptions[$endHandle]);
             throw $exception;