Added selective field getters for JsObject

[cvrabie]

When writing a custom JsonFormat the currently proposed way of using pattern matching/extractors on JsObject.getFields is elegant and concise especially when all the fields are required.

However we often meet the situation where, even though the fields in the domain object are required, we can provide default values for some of these fields, thus making some part of the incoming json optional.

For example, a User class might require a language and timezone parameters but we we can use the system defaults if the incoming json does not specify these fields.

I experimented with multiple options that would make easier the writing of JsonFormats that allow optional fields and I think the best one is to add specialised field getters in the JsObject class. Then flatMap/map or a for comprehension can be used in the JsonFormat to extract the fields as demonstrated in the test.

[cvrabie]

This can be done better with the decorator pattern. I'll close and re-submit.

[jrudolph]

Actually, there's already a version 2 of spray-json in the work which has support for optional / default parameters. See 7352db2#L5R86

Unfortunately, we currently have no time to finish that work before spray 1.0 is released.

[cvrabie]

Is there any way I might help with that? It doesn't look like rocket science in itself though I'm not sure how many dependencies it has.

[cvrabie]

@jrudolph by the way, i refactored it so it doesn't change any of your classes. You can just drop the rich wrapper in a project and you can use it: #64