Fix serialization of classes extending \yii\symfonymailer\Message

[Eseperio]

Current visibility of properties $email and $charset (private) limits the ability to serialize any inherited class, since __sleep(), when is called in a subclass, cannot access those properties and thus serialization is missing the most important part of class, the email. Changing $email and $charset to protected addresses this.

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.14%. Comparing base (d843854) to head (240ff21).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #82      +/-   ##
============================================
+ Coverage     85.07%   85.14%   +0.07%     
  Complexity       71       71              
============================================
  Files             4        4              
  Lines           201      202       +1     
============================================
+ Hits            171      172       +1     
  Misses           30       30              

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[samdark]

@SamMousa would you please take a look?

[SamMousa]

1. __sleep() is deprecated. https://wiki.php.net/rfc/soft-deprecate-sleep-wakeup
2. I don't think you should be extending framework classes, it is an anti-pattern
3. If you really need to serialize your dependent class I'd accept a PR that implements Serializable in the Message class and removes the __sleep() implementation, so that you could use parent::serialize() in your own class.
4. Both the properties you mention have public getters...

Making stuff more open is usually not the way forward. I'd even argue that this class should be final.

[samdark]

Implementing Serializable is a very good idea.