Improve function naming in `URI`
Make `URI.encode/1` to conform to RFC 3986
Introduce `URI.encode/2` function

`URI.encode/1` escapes character if it
is not satisfied `URI.char_unescaped?/1`
Merge pull request #2392 from lexmag/improve-uri

Make `URI.encode/1` to conform to RFC 3986. Introduce `URI.encode/2` function.