Fix jsonNode#addAll

[heshamMassoud]

Addresses #2442

[cowtowncoder]

Looks good, thank you for contributing the fix!

Just one process thing before I will merge this: before the first contribution, we need a CLA filled; it can be found from:

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

and it's usually easiest to print, fill & sign, scan (cell phone photo works) and email to info at fasterxml dot com. Once this is done (and it is only needed once -- further contributions to all Jackson repos are covered), I'll be happy to merge the fix.

Once again thank you.

[heshamMassoud]

@cowtowncoder filled and emailed 😉