filter generator: create child object context when writing start object. fixes #890

[DemonicTutor]

Fixes #890.

[cowtowncoder]

Thank you @DemonicTutor! Happy to merge this ASAP.

Just one clerical thing: have I yet asked for a CLA? If not, it's here

https://github.com/FasterXML/jackson/blob/master/contributor-agreement.pdf

(and alternatively Corporate CLA if preferred)

and the usual mechanism is to print it, fill & sign, scan/photo, email to info at fasterxml dot com.
Only needs to be done once for all future contributions.
(also if you have sent one earlier let me know; I didn't see one but might have missed it).

Looking forward to merging this!

[cowtowncoder]

Oh also, not sure if it's easy to rebase but this could be merged into 2.14 branch. Or I can try to cherry-pick after merge to 2.15.

[DemonicTutor]

@cowtowncoder hey! i sent you the requested document from my company mail account.

yeah if you are happy with this one as-is i can also open a Pull-Request against 2.14?

[cowtowncoder]

Thanks, I can see that now. And yes, the other request is fine; could have rebased this one but separate PR is ok too.