Unit Test and Bug Fix for Issue 4

Submitting a PR containing the following changes: 

* A unit test confirming issue https://github.com/greenmail-mail-test/greenmail/issues/4
* An update to the JavaMail dependency (latest version is 1.5.2 - we're using 1.4.7)
* An additional helper method in GreenMailUtil to accept a MimeMessage object to send to GreenMail SMTP
* An inner class in GreenMailUtilTest (should probably be relocated to a test helper class) that extends MimeMessage and sets the message-id. JavaMail by default will set the message-id for each message it sends. See the following [JavaMail FAQ] (http://www.oracle.com/technetwork/java/faq-135477.html#msgid) for more details