Allow POST, PATCH, DELETE for actions in ReadOnlyModelViewSet (django-json-api#797)

Currently if you try to use `POST` for action in `ReadOnlyModelViewSet` you will get problems:
- with DRF UI you will loose data input forms
- `drf_yasg` package will not generate OpenAPI specifications for such actions

This behavior was added to `django-rest-framework-json-api` in version 2.8.0 by mistake:

Commit: 7abd764
Subject: remove disallowed PUT method. (django-json-api#643)

Author: kolomenkin

rest_framework_json_api/views.py

@@ -210,7 +210,7 @@ class ModelViewSet(AutoPrefetchMixin,
 class ReadOnlyModelViewSet(AutoPrefetchMixin,
                            RelatedMixin,
                            viewsets.ReadOnlyModelViewSet):
-    http_method_names = ['get', 'head', 'options']
+    http_method_names = ['get', 'post', 'patch', 'delete', 'head', 'options']
 
 
 class RelationshipView(generics.GenericAPIView):