Add Postmark Message ID to response headers (#49)

Pulls in changes from #46 (thank you @dejury, and sorry for the wait! 🙇 ) and adds a test for it.

Closes #46 #24 (had to rebuild the PR after the repo move).

README.md

@@ -79,6 +79,20 @@ $message = new Swift_Message('Hello from Postmark!');
 $message->getHeaders()->addTextHeader('X-PM-Message-Stream', 'another-stream');
 ```
 
+##### 6. Getting the Postmark Message ID after sending
+After sending the mail to Postmark, it is possible to get the Postmark ID.
+
+```php 
+$transport = new \Postmark\Transport('<SERVER_TOKEN>', $defaultHeaders);
+$mailer = new Swift_Mailer($transport);
+
+$message = new Swift_Message('Hello from Postmark!');
+
+$mailer->send($message);
+
+$postmarkID = $message->getHeaders()->get('X-PM-Message-Id')->getValue();
+```
+
 ##### Notes:
 
 - The Transport uses the [Postmark API](https://postmarkapp.com/developer) internally to send mail, via the [/email endpoint](https://postmarkapp.com/developer/api/email-api#send-a-single-email). Other sending features such as Batch sending or sending via Templates are currently not supported by this library.

src/Postmark/Transport.php

@@ -108,6 +108,12 @@ public function send(Swift_Mime_SimpleMessage $message, &$failedRecipients = nul
 			$this->_eventDispatcher->dispatchEvent($responseEvent, 'responseReceived');
 		}
 
+		// Get the Postmark message ID
+		$jsonResponse = json_decode($response->getBody()->__toString(), true);
+		if (isset($jsonResponse['MessageID'])) {
+		    $message->getHeaders()->addTextHeader('X-PM-Message-Id', $jsonResponse['MessageID']);
+		}
+
 		if ($sendEvent) {
 			$sendEvent->setResult($success ? \Swift_Events_SendEvent::RESULT_SUCCESS : \Swift_Events_SendEvent::RESULT_FAILED);
 			$this->_eventDispatcher->dispatchEvent($sendEvent, 'sendPerformed');

tests/TransportTest.php

@@ -214,6 +214,18 @@ public function testPmTagDeaultHeader()
 
     $this->assertEquals('my-tag', $body['Tag']);
   }
+
+  public function testMessageIdDefaultHeader()
+  {
+    $message = new Swift_Message();
+
+    $transport = new PostmarkTransportStub([new Response(200, [], '{"MessageID": "MESSAGE_ID"}')]);
+    $transport->send($message);
+
+    $postmarkID = $message->getHeaders()->get('X-PM-Message-Id')->getValue();
+
+    $this->assertEquals($postmarkID, "MESSAGE_ID");
+  }
 }