add test case for invalid characters in cert commonnames

Hey @cortesi,

while <code>check_domain</code> allows all plain-ASCII chars in the commonname, some operating systems or file systems (Windows/FAT) don't. This has the nasty side effect that wildcart certificates (<code>\*.facebook.com, \*.github.com</code>) break. I am using <code>.replace("*","")</code> as a nasty workaround, but you mentioned that you're looking for a more generic solution here. Let me know what you're thinking of and I'll give it a shot.

Cheers,
Max