support nil charset

noticed nil failing when dropping the 772 fix into our mail parser project

@bf4 
typo: s/bicks/picks

I don't see why you wouldn't make this test equivalent to the other `expect { ::Encoding.find(from) }.to raise_error(TypeError)`. or also test `picks binary for empty string`.

I don't want that to block the bugfix, but, to me, those are both contracted behaviors to test.