! httpx: UTF-8 % encode FormData by default

Encode form data to utf-8 by default. I think this is a bit subjective but reading the RFC (and even the test case) i'd expect it to be the default behavior. 

http://tools.ietf.org/html/rfc3986#section-2.1
Not sure this was ever valid: `h%E4llo` is not valid UTF-8 % encoding.