Support punycode

I'm not sure about the details, but it might be nice to allow URIs of non-ASCII text (i.e. use `String` rather than `ASCIIString` in the type definition). The RFC is a bit vague on this. Perhaps it isn't technically allowed but it seems possible to encounter in the wild (e.g. ☃.net will resolve in a browser). On a practical level it's somewhat annoying to not be able to pass `UTF8String`s or `SubString`s thereof to methods in Requests.jl.