UTF8 encoded subjects

[donmhico]

Description

This PR adds the ability to support UTF8-encoded subjects in the email logs table

Testing Procedure

1. Submit an email using wp_mail() with subject containing non ASCII characters. You can use the snippet below.

   add_action( 'init', function () {
   $to      = 'donmhico+test1@gmail.com';
   $subject = 'Subject with non ASCII ó¿¡á';
   $message = 'Message with non ASCII ó¿¡á';
   $headers = 'From: example@example.com'."\r\n"
   .'Content-Type: text/plain; charset=utf-8'."\r\n";
   
   wp_mail(
       $to,
       '=?utf-8?B?'. base64_encode( $subject ) . '?=',
       $message,
       $headers
   );
   } );

2. Check WP Mail Logging's Email Logs. You should see the subject displaying properly.

If you see the log details, you should also see this in HTML tab.

However, you should see the actual encoded subject in both RAW and JSON tab.