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've started using a simple "Location" struct like this:

`{"name": "London", "lat": 51.507222, "lon": -0.1275}"`

Would this one be ok?