Not dropping too many null values in OpenAPI encoders

[ghik]

Fixes #206

[ghik]

@adamw I realized that there are other pitfalls - even dropping null field values recursively is not safe, because the OpenAPI may contain objects that must be retained verbatim, e.g. an example value in a schema with null-valued fields.

To me the right approach seems to drop null fields carefully and separately (and non-recursively) in each encoder. I've updated the PR to do that.

[adamw]

Thanks for the changes :) Some tests are failing now, though

[ghik]

@adamw all seems green now :)

[adamw]

Awesome, thanks! :)