MailServiceImpl: add unit tests

[php-coder]

MailServiceImpl class doesn't have any unit tests. We should add them.

Technical Debt for: 681913b (#1)

[php-coder]

To simplify unit testing and to properly fix the following warnings from PMD:

[INFO] PMD Failure: ru.mystamps.web.service.MailServiceImpl:135 Rule:AccessorMethodGeneration Priority:3 Avoid autogenerated methods to access private fields and methods of inner / outer classes.
[INFO] PMD Failure: ru.mystamps.web.service.MailServiceImpl:141 Rule:AccessorMethodGeneration Priority:3 Avoid autogenerated methods to access private fields and methods of inner / outer classes.

I suggest to extract MimeMessagePreparator implementation to a separate class.

[php-coder]

I suggest to extract MimeMessagePreparator implementation to a separate class.

It has been extracted in cacc3cc (as part of #935)