Fix SMTP sender verification after WordPress changes

WordPress changeset 61010 removed the third parameter (false) from
setFrom() in wp_mail(), which now causes the Sender header to be
automatically set. This can cause sender verification failures with
some SMTP servers.

This fix explicitly sets Sender property to match From address.

Fixes #15

Author: ribvid

src/AcornMail.php

@@ -64,6 +64,8 @@ protected function configureMail(): void
                 $this->fromAddress(),
                 $this->fromName()
             );
+
+            $mail->Sender = $mail->From;
         });
     }