Implement a ReadJSON() utility to help reduce some code-duplication,
and to make sure we handle JSON requests consistently (e.g. always
check for the content-type).
Differences compared to current handling:
- prevent possible panic if request.Body is nil ("should never happen")
- always require Content-Type to be "application/json"
- be stricter about additional content after JSON (previously ignored)
- but, allow the body to be empty (an empty body is not invalid);
update TestContainerInvalidJSON accordingly, which was testing the
wrong expectation.
- close body after reading (some code did this)
We should consider to add a "max body size" on this function, similar to
7b9275c0da/api/server/middleware/debug.go (L27-L40)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>