Adding utils\net\SMTP\Client\Message tests

I could not make tests for when message isn't provided because get a error due to construtor $message parameter is empty.

This can be solved setting default property value in constructor, instead of directly in property declaration, as in this commit: https://github.com/tsouzar/SMTPClient/commit/0bb45748499c1ae3e7478e4de5d8306f6ac68e4c

After change this I will can create the remaining tests.