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

lib/active_model_serializers/adapter/json_api/resource_identifier.rb

@@ -2,11 +2,30 @@ module ActiveModelSerializers
   module Adapter
     class JsonApi
       class ResourceIdentifier
+        def self.type_for(class_name, serializer_type = nil, transform_options = {})
+          if serializer_type
+            raw_type = serializer_type
+          else
+            inflection =
+              if ActiveModelSerializers.config.jsonapi_resource_type == :singular
+                :singularize
+              else
+                :pluralize
+              end
+
+            raw_type = class_name.underscore
+            raw_type = ActiveSupport::Inflector.public_send(inflection, raw_type)
+            raw_type
+              .gsub!('/'.freeze, ActiveModelSerializers.config.jsonapi_namespace_separator)
+            raw_type
+          end
+          JsonApi.send(:transform_key_casing!, raw_type, transform_options)
+        end
+
         # {http://jsonapi.org/format/#document-resource-identifier-objects Resource Identifier Objects}
         def initialize(serializer, options)
           @id   = id_for(serializer)
-          @type = JsonApi.send(:transform_key_casing!, type_for(serializer),
-            options)
+          @type = type_for(serializer, options)
         end
 
         def as_json
@@ -19,13 +38,8 @@ module ActiveModelSerializers
 
         private
 
-        def type_for(serializer)
-          return serializer._type if serializer._type
-          if ActiveModelSerializers.config.jsonapi_resource_type == :singular
-            serializer.object.class.model_name.singular
-          else
-            serializer.object.class.model_name.plural
-          end
+        def type_for(serializer, transform_options)
+          self.class.type_for(serializer.object.class.name, serializer._type, transform_options)
         end
 
         def id_for(serializer)