toJSONTag should conform to toJSON method

[poef]

toJSON() is defined such that you return a value, which is then given to JSON.stringify(), it does not return a string.

toJSONTag() now returns a string, which should be valid JSONTag, but there is no check.
Instead it should return a value, just like toJSON(), and JSONTag should then stringify() it. This way, the result will always be valid JSONTag.

In addition, the default toJSONTag() functions expect toJSON() to return a string, which is incorrect.

[poef]

Solved in release 0.9.11