Error raising when charset is missing

[janko]

First of all, thank you for an awesome gem. So far I found it to be much better than rack-test.

One issue that I've encountered was, if the response Content-Type is e.g. "text/html", Rack::TestApp will raise an error about missing charset. I don't think that's ideal, because AFAIK from HTTP standpoint a charset isn't required. I was thinking instead to have the charset simply default to "utf-8", because that's Ruby's default encoding anyway, and it's very likely that this is what the user would want.