[V3] JsonSerializable should avoid returning concrete types in some cases

[christopheranderson]

JsonSerializable has a few interesting return types. getHashMap() returns a HashMap, when one would expect it to be Map, and the getCollection(...) methods return Collection, when one might expect a List or Set.

(Collection should actually be fine as it is an interface)

Let's go with renaming the methods and fixing return types

• getHashMap(): HashMap should now be getMap(): Map

[christopheranderson]

done