Canonical JWT encoding

From perl v5.18, the key order of hash is randomized and two JSON encoded string may be different even if they come from same structure of hash.

Encoding to JWT has same problem.

```
#!/usr/bin/env perl
use v5.18;
use JSON::WebToken;

for (1..3) {
    my $claims = {
        sub => '1234',
        iss => 'dave',
        aud => 'tom',
    };

    say encode_jwt $claims, 'secret'; 
}
```

This simple sample code says
```
eyJhbGciOiJIUzI1NiJ9.eyJhdWQiOiJ0b20iLCJzdWIiOiIxMjM0IiwiaXNzIjoiZGF2ZSJ9.bajTuYDNPetfv_Zb3OGDgSutIDq5HY6aKC9H9y1PfD4
eyJhbGciOiJIUzI1NiJ9.eyJzdWIiOiIxMjM0IiwiaXNzIjoiZGF2ZSIsImF1ZCI6InRvbSJ9.z2lVlConVX2YKT7xrpe4EE7IneVQTFcRjtqPxpT_xjM
eyJhbGciOiJIUzI1NiJ9.eyJpc3MiOiJkYXZlIiwic3ViIjoiMTIzNCIsImF1ZCI6InRvbSJ9.104dgCS4l4l49igJ4Z07PA3hVp_AGQmdTRe5uB2h2Kc
```
Encoding results are completely deferent strings even if they have same claims.

[JWT spec](https://tools.ietf.org/html/draft-ietf-oauth-json-web-token-30) doesn't refer to above things but I think the same JWT claims should have exactly same JWT string.

This pull-req is intended to fix that problem to use canonical JSON encoding.