Percent encode (url encode) an utf8 string, as specified in RFC 3986 section 2.1

* The alphanumeric characters "a" through "z", "A" through "Z" and "0" through "9" remain the same.
* The special characters "-", ".", "_", and "~" remain the same.
* All other characters are unsafe, their bytes are individually converted to a 3-character string "%HH",
where HH is the two-digit hexadecimal representation of the byte. This also applies to the space character.