Make flush really async and use promise wait accurately (#212)

* Fix async issue

* Remove unused reference var

Author: marcvdm

src/WebPush.php

@@ -16,6 +16,7 @@
 use Base64Url\Base64Url;
 use GuzzleHttp\Client;
 use GuzzleHttp\Exception\RequestException;
+use GuzzleHttp\Promise\Promise;
 use GuzzleHttp\Psr7\Request;
 use Psr\Http\Message\ResponseInterface;
 
@@ -166,22 +167,23 @@ public function flush(?int $batchSize = null) : iterable
 	        // for each endpoint server type
 	        $requests = $this->prepare($batch);
 
-	        foreach ($requests as $request) {
-	        	$result = null;
+            $promises = [];
 
-		        $this->client->sendAsync($request)
-			        ->then(function ($response) use ($request, &$result) {
+	        foreach ($requests as $request) {
+                $promises[] = $this->client->sendAsync($request)
+			        ->then(function ($response) use ($request) {
 				        /** @var ResponseInterface $response * */
-				        $result = new MessageSentReport($request, $response);
+				        return new MessageSentReport($request, $response);
 			        })
-			        ->otherwise(function ($reason) use (&$result) {
+			        ->otherwise(function ($reason) {
 				        /** @var RequestException $reason **/
-				        $result = new MessageSentReport($reason->getRequest(), $reason->getResponse(), false, $reason->getMessage());
-			        })
-			        ->wait(false);
-
-		        yield $result;
+				        return new MessageSentReport($reason->getRequest(), $reason->getResponse(), false, $reason->getMessage());
+			        });
 	        }
+
+	        foreach ($promises as $promise) {
+	            yield $promise->wait();
+            }
         }
 
         if ($this->reuseVAPIDHeaders) {