Add '_' to url unreserved chars list

According to rfc2396 underscore is not reserved
http://tools.ietf.org/html/rfc2396.html#section-2.3

Though technically escaping everything is correct, this produces "better looking" urls

Also [section 3.3](http://tools.ietf.org/html/rfc2396.html#section-3.3) has more characters which don't need to be escaped in path segment.
`application/x-www-form-urlencoded` is different beast though.