Namespace separator setting for json-api and tests (#1874)

Adds jsonapi_namespace_separator configuration

Also:

* Enable getting type from record class without serializer

Needs Followup:

- https://github.com/rails-api/active_model_serializers/pull/1874#discussion_r74607042 
- https://github.com/rails-api/active_model_serializers/pull/1874#discussion_r74607734

lib/active_model/serializer/configuration.rb

@@ -21,13 +21,14 @@ module ActiveModel
 
         config.default_includes = '*'
         config.adapter = :attributes
+        config.key_transform = nil
         config.jsonapi_resource_type = :plural
+        config.jsonapi_namespace_separator = '-'.freeze
         config.jsonapi_version = '1.0'
         config.jsonapi_toplevel_meta = {}
         # Make JSON API top-level jsonapi member opt-in
         # ref: http://jsonapi.org/format/#document-top-level
         config.jsonapi_include_toplevel_object = false
-        config.key_transform = nil
 
         config.schema_path = 'test/support/schemas'
       end