Fix empty relationships on included resources (#1372)

JSONAPI::ResourceTree#load_included was failing to correctly
populate the resource fragments of included resources, such that
the `include_related` parameter was null. This was resulting in
the relationships object lacking a data attribute for nil or
empty resources.

The JSON-API specification states that a null or empty array should
be returned in these circumstances:
https://jsonapi.org/format/#document-resource-object-linkage

The underlying issue appears to be the use of `include_related` rather
than the symbol `:include_related` when initializing nested resource
fragments.

Author: JamesGlover

lib/jsonapi/resource_tree.rb

@@ -85,7 +85,7 @@ def load_included(resource_klass, source_resource_tree, include_related, options
                                                                    find_related_resource_options)
 
         related_resource_tree = source_resource_tree.get_related_resource_tree(relationship)
-        related_resource_tree.add_resource_fragments(related_fragments, include_related[key][include_related])
+        related_resource_tree.add_resource_fragments(related_fragments, include_related[key][:include_related])
 
         # Now recursively get the related resources for the currently found resources
         load_included(relationship.resource_klass,

test/controllers/controller_test.rb

@@ -4371,7 +4371,8 @@ def test_complex_includes_things_nested_things
                             "links" => {
                                 "self" => "http://test.host/api/things/40/relationships/things",
                                 "related" => "http://test.host/api/things/40/things"
-                            }
+                            },
+                            "data"=>[]
                         }
                     }
                 },