[FIXED JENKINS-20226] Supports @DataBoundConstructor.

Supports `@DataBoundConstructor`, which is the modern Jenkins way to instantiate from form submit.
This makes Mailer plugin work with [Flexible Publisher plugin](https://wiki.jenkins-ci.org/display/JENKINS/Flexible+Publish+Plugin), which is reported on [JENKINS-20226](https://issues.jenkins-ci.org/browse/JENKINS-20226).