Make message class inheritable again

[mtangoo]

Some key properties of the class Message are private preventing any proper inheritance.
Open them up by making them protected

Q	A
Is bugfix?	✔️
New feature?	❌
Breaks BC?	❌
Fixed issues	#58

[what-the-diff]

PR Summary

• Increased Access Level for Certain Properties in 'Message' Class
  The properties related to email and character set in the 'Message' class were restricted to be accessed within the class only. This change will allow these properties to be accessible in any class that extends from the 'Message' class itself thereby providing more flexibility in manipulating this information.