json: add sub-section on default de/enconding (staging area)

Cover deconding from &str, default decoding and encoding into String (ref. #86)

(Similar to the previous PR, with added encoding)
I prefer to not use the `unwrap` method. It's preferable to expliclty deal with the failure path (yes, I know this won't fail in this example), and tell the reader how could the operation fail, and it'll be best to show the reader an example string that fails at this step (malformed json: missing brace/comma, etc).