fixup: Preserve nil links

Just as nil attributes are still serialized, this commit ensures that nil links
are also serialized. If nil links are not serialized, Spraypaint does not remove
the old link value after an update operation.

Author: superslau

lib/graphiti/serializer.rb

@@ -53,7 +53,7 @@ def respond_to_missing?(method_name, include_private = true)
     def add_links!(hash)
       return unless @resource.respond_to?(:links?)
 
-      hash[:links] = @resource.links(@object).compact if @resource.links?
+      hash[:links] = @resource.links(@object) if @resource.links?
     end
 
     def strip_relationships!(hash)

spec/serialization_spec.rb

@@ -1405,9 +1405,10 @@ def classification
         resource.link :test_link do |model| nil end
       end
 
-      specify "are omitted" do
+      specify "are still included" do
         render
-        expect(json["data"][0]["links"]).not_to have_key("test_link")
+        expect(json["data"][0]["links"]).to have_key("test_link")
+        expect(json["data"][0]["links"]["test_link"]).to eq(nil)
       end
     end
   end