escaped characters in the URI are not honored

If we have a mapping rule like /foo/

/baz
when we run curl "apicast.url/foo/bar%20%20bar/baz" we get a 404 error.

This is because here(https://github.com/3scale/APIcast/blob/68faa71/gateway/src/apicast/proxy.lua#L245) we use ngx.var.uri that unescape HTML chars.