One of three constituents is used to provide the
CollectionSerializer's #json_key:
1) the :root option - controlled by the caller
2) the #name of the first resource serializer - the root or
underscored model name
3) the underscored #name of the resources object - generally
equivalent to the underscored model name of #2
Of the three, only the latter 2 are out of the callers control, and
only the latter two are expected to be singular by default. Not
pluralizing the root gives the caller additional flexibility in
defining the desired root, whether conventionally plural,
unconventionally plural (e.g. objects_received:) or singular.
* Use assert_nil where appropriate
* Lead with the expected value in collection_serializer_test.rb, etc
so that expected/actual in test failure messages are not reversed
