fix: Reject invalid MIME header name characters to support Go 1.20

[jhillyerd]

Fixes #269

[kayabe]

test fails because of this commit golang/go@a6642e6, = doesnt seem to be permitted

[jhillyerd]

Yes, as seems to be the usual with enmime, we have to decide how to handle malformed email in an elegant way.

It does not appear there is an official way to escape characters in mime header names. The easiest would probably be to convert the equals to a dash. I could imagine that being exploited, so perhaps a double dash would be better. Might also provide a strict mode to discard that particular header line instead.

[rjbordon]

Hi @jhillyerd, first of all, thanks for this awesome library!

I have been testing the latest and greatest in the master branch (this PR included) with go1.20.1 and it works out as expected. Could you kindly create a new release from the master branch? I think the last one available goes until Sept 2022.

Thanks.

[jhillyerd]

done, v0.11.0 tagged

[rjbordon]

Thank you!