Adds tests to the mailer plugin

Some attributes are protected/private in HtmlEmail/MultiPartEmail. Apache commons-email use mock to expose these attributes:
 * [MockHtmlEmailConcrete](https://github.com/apache/commons-email/blob/trunk/src/test/java/org/apache/commons/mail/mocks/MockHtmlEmailConcrete.java)
 * [MockMultiPartEmailConcrete](https://github.com/apache/commons-email/blob/trunk/src/test/java/org/apache/commons/mail/MultiPartEmailTest.java)

I think we can do the same thing by mocking `createEmailBody` method.