Serialize null values in all maps

[ob-stripe]

r? @brandur-stripe @richardm-stripe
cc @stripe/api-libraries

This PR serializes null values in all Maps when converting typed parameters classes to untyped maps.

Previously, we were only serializing null values in Map<String, String> maps. The intention was that only metadata fields had this type and that's where we wanted to serialize values.

However, after #849, some metadata fields (such as TransactionUpdateParams.metadata) were no longer typed as Map<String, String> but rather as Object because they became polymorphic (either a HashMap<String, String> or an EmptyParam).

Due to type erasure, the type parameters are no longer present at runtime and the type adapter that was supposed to serialize null values failed to do so because this check failed.

This PR simplifies the type adapter by simply checking if the value is a Map (or any subclass such as HashMap). When that's the case, the type adapter sets serializeNulls(true) and then delegates to the normal adapter for the type. As a result, all null values are now serialized in all Maps. I think that's what we want anyway: if an entry is present in a map, it was necessarily added by the user and should be serialized.

Fixes #893.

[brandur-stripe]

if an entry is present in a map, it was necessarily added by the user and should be serialized.

Yeah, I would've said the same thing.

Thanks for the detailed PR description and for tracking all of this down. LGTM.

[ob-stripe]

Released as v15.3.1.