router: Don't escape Request-URI

net/http uses the RequestURI method of the url to write the
Request-URI field. By default this escapes the Path field, which is
stored unescaped. By using the Opaque field we can work around this
and pass the Request-URI we got through to the backend without
modification.

Fixes #177

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>