Support for nullable/x-nullable

[kbedel]

Fixes #300

Currently these properties are always going to be written. If you think this could cause issues, I can add support for it being behind a flag.

[kbedel]

@kailuowang Would it be possible to get this merged? I can also create an equivalent PR for Play 2.8 if you like

[kailuowang]

@kbedel sorry I lost track of this one. I am not sure that you need an extra field in all those places. Is it not possible to just use the required field? I mean if it's not required you add an x-nullable property?

[kbedel]

OpenAPI Generator requires the x-nullable attribute to properly generate the C# client code we are using (the issue has links to their doc and a closed issue from that side)

[kbedel]

@kailuowang Do you also need a play 2.8 version in order for this to get merged? We currently are relying on a forked version but I'd like to move back to the mainline as we move to Play 2.8

[kailuowang]

@kbedel yes if you don't mind please create a separate PR to the 2.8 branch. I'll merge both and release.