Email can have custom headers

This minor release ships a minor feature that enable to set custom headers in the email.

the new method to be used for the Email interface is setCustomHeaders.
For the default implementation, you can also use the builder.

Map<String, String> headers = ...

final Email email = DefaultEmail.builder()
        .from(new InternetAddress("cicero@mala-tempora.currunt", "Marco Tullio Cicerone "))
        .to(Lists.newArrayList(new InternetAddress("titus@de-rerum.natura", "Pomponius Attĭcus")))
        .subject("Laelius de amicitia")
        .body("Firmamentum autem stabilitatis constantiaeque eius, quam in amicitia quaerimus, fides est.")
        .customHeaders(headers)
        .encoding(Charset.forName("UTF-8")).build();