Properly serialize zero-value messages in a oneof group (#176)

Also improve test utils to make it easier to have multiple json examples.

Co-authored-by: Christopher Chambers <chris@peanutcode.com>

src/betterproto/__init__.py

@@ -699,7 +699,7 @@ class Message(ABC):
                     meta.number,
                     meta.proto_type,
                     value,
-                    serialize_empty=serialize_empty,
+                    serialize_empty=serialize_empty or selected_in_group,
                     wraps=meta.wraps or "",
                 )