Mapping keys can be string, dont have to be identifier

• Changed SPEC to allow mapping-keys to be Unicode strings.
• Update all implementations to allow this.
• Update shared test to ensure that implementations can do this.
• NOTE: while ok in Octave, a comment in the json encoder suggests that Matlab does not, but if I want to test this I need to get an evaluation copy, using a different email address that I used last time, and ... nobody uses Matlab anymore anyway, right?
• Updated JS implementation to fail early when undefined is given, instead of letting the extension have a try, and thereby failing with an undesciptive error message.