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)
You should end this line with a comma. This is one of the things I really like about Rust is that the extra comma is not an error. Ending all these lines with commas, not only makes the code more symmetric, but also improves the output of `diff` commands.