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

test/adapter/json_api/linked_test.rb

@@ -223,6 +223,25 @@ module ActiveModelSerializers
           assert_equal expected, relationships
         end
 
+        def test_underscore_model_namespace_with_namespace_separator_for_linked_resource_type
+          spammy_post = Post.new(id: 123)
+          spammy_post.related = [Spam::UnrelatedLink.new(id: 456)]
+          serializer = SpammyPostSerializer.new(spammy_post)
+          adapter = ActiveModelSerializers::Adapter::JsonApi.new(serializer)
+          relationships = with_namespace_separator '--' do
+            adapter.serializable_hash[:data][:relationships]
+          end
+          expected = {
+            related: {
+              data: [{
+                type: 'spam--unrelated-links',
+                id: '456'
+              }]
+            }
+          }
+          assert_equal expected, relationships
+        end
+
         def test_multiple_references_to_same_resource
           serializer = ActiveModel::Serializer::CollectionSerializer.new([@first_comment, @second_comment])
           adapter = ActiveModelSerializers::Adapter::JsonApi.new(