Fix snake case deserialization

[juulhobert]

Issue: #176

[cowtowncoder]

First of all, thank you @juulhobert for working on this! I know it's a draft and may take some time.

Eventually if all goes well, we'll need CLA (assuming you haven't sent one earlier): only needs to be done once (good for all future contributions)

It's here:

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

and is usually printed, filled & signed, photo/scanned, emailed to cla at fasterxml dot com.

But this is only need to merge, just mentioning it now.

Will also have a look at changes now.