Add JsonGeneratorDecorator to allow decorating JsonGenerators

[digulla]

Support to register decorators for JsonGenerator in the builder and factory.

[cowtowncoder]

Cool, I like this. Let me think a bit about it, just to be sure, but I think this could work about as-is.

[digulla]

One thing that comes to my mind: Is there anything else in the factory which could use this?

Also, is the _decorate() method in the right class? It could go into TSFBuilder instead.

[cowtowncoder]

Forgot to ask: any chance for a simple test or two, to verify decoration actually works...

[cowtowncoder]

Looks good overall; added notes but will address them post-merge.

[cowtowncoder]

One thing that comes to my mind: Is there anything else in the factory which could use this?

Also, is the _decorate() method in the right class? It could go into TSFBuilder instead.

I think it's in the right class -- it could not really go in factory builder (as that is only part of constructing factory itself, not generator).

One bigger thing we should tackle is to change all format backends to allow decoration. In 3.0 (master) this is relatively easy due to full refactoring of TokenStreamFactory hierarchy. But in 2.x there's much more duplication so pretty much every XxxFactory needs to call _decorate().

I did at least add tests for JsonFactory, fwtw. Changing other backends is not difficult, just lots of work.