Fix reply-to address when sending WP e-mails

This commit makes sure that the reply-to set for WP e-mails is
preserved when MailPoet is used to send them. Before this change,
MailPoet would ignore whatever was set for the reply-to field for WP
e-mails and use the same address as the one used for the sender instead.

I'm not super happy with the solution proposed here. I couldn't find an
easy way to simple set the reply-to address in the Mailer class and that
is why I'm calling Mailer::init() inside WordPressMailer.

[MAILPOET-3707]

lib/Mailer/WordPress/WordPressMailer.php

@@ -51,7 +51,15 @@ class WordPressMailer extends \PHPMailer {
     ];
 
     $sendWithMailer = function ($mailer) use ($email, $address, $extraParams) {
+      // we need to call Mailer::init() for every single WP e-mail to make sure reply-to is set
+      $replyTo = $this->getReplyToAddress();
+      $mailer->init(false, false, $replyTo);
+
       $result = $mailer->send($email, $address, $extraParams);
+
+      // make sure Mailer::init() is called again to clear the reply-to address that was just set if Mailer is used in another context
+      $mailer->mailerInstance = null;
+
       if (!$result['response']) {
         throw new \Exception($result['error']->getMessage());
       }
@@ -103,4 +111,25 @@ class WordPressMailer extends \PHPMailer {
     }
     return $result;
   }
+
+  private function getReplyToAddress() {
+    $replyToAddress = false;
+    $addresses = $this->getReplyToAddresses();
+
+    if (!empty($addresses)) {
+      // only one reply-to address supported by \MailPoet\Mailer
+      $address = array_shift($addresses);
+      $replyToAddress = [];
+
+      if ($address[1]) {
+        $replyToAddress['name'] = $address[1];
+      }
+
+      if ($address[0]) {
+        $replyToAddress['address'] = $address[0];
+      }
+    }
+
+    return $replyToAddress;
+  }
 }