Default initialization for vmime::mailbox

[jengelh]

Testcase:

#include <vmime/mailbox.hpp>
int main()
{
        vmime::mailbox mb;
        printf(">%s<\n", mb.generate().c_str());
        mb.setEmail("");
        printf(">%s<\n", mb.generate().c_str());
        return 0;
}

Observed output:

>@<
>invalid@invalid<

Expected output:

>invalid@invalid<
>invalid@invalid<

What do you think? Should the default ctor produce something that is at least syntactically valid?