Support application/x-www-form-urlencoded when request body is Dict/Vector{Pair}

[quinnj]

This is supported in most client http libraries out there, but we don't really support it. We support the more complicated HTTP.Form case for multipart stuff, but not the simple form-urlencoded. Currently if you pass a Dict as the request body, you get this weird error about how it can't write Pair to a Stream, so it seems ripe for providing a more useful default behavior for users who may be coming from other implementations.