I incorporated that change as well.  My rationale behind the previous version was to be consistent with how Lib/email/header.py handled this, unfortunately I did not look around in the other classes and didn't think about that kind of compatibility.

When formataddr() is called with a object which is not a string and which does not have a header_encode it will raise the following exception now:

> AttributeError: 'CharsetMock' object has no attribute 'header_encode'

Thank you for your patience, sorry that it took probably more of your time by taking 4 iterations for this patch than if you had just implemented it yourself.