json Utf8 behavior

Coming from #771 

In a nutshell, we make Dancer's from_json behave like 'decode_json', but don't do the reverse. Hilarity ensues.

This fix document our helpful behavior, and make the from_json and to_json.

Now, I'm kinda scared of the comment that was there for 'encode_json', to the effect that the utf8 encoding would be done down the line. I still have to make due dilligences to make sure this fix doesn't inject any double-encoding bug.