Fix mimetypes are always text/plain in mock mode.

Currently when I send an email as follows:

    HtmlEmail email = new HtmlEmail();
    email.setSubject("hi");
    email.setTextMsg("hello");
    email.setHtmlMsg("<p>hello</p>");
    Mailer.send(email);
    
If smtp.mock is true, the debug message says the second entity body's content type is text/plain even if it is text/html.

    [debug] application - mock email send:
    From: foo@gmail.com
    To:   bar@gmail.com
    Date: Fri Jul 12 16:42:43 KST 2013
    Subj: hi
    cont:
    text/plain: hello
    text/plain: <p>hello</p>

In order to fix this, saveChanges() is need to be called after calling buildMimeMessage() in getEmailDebugOutput(), to set contet types correctly.