Transform relationship keys with shallow field transformation options

lib/jsonapi/utils/string.ex

@@ -261,6 +261,10 @@ defmodule JSONAPI.Utils.String do
     end)
   end
 
+  def expand_root_keys(key, fun) when is_binary(key) or is_atom(key) do
+    fun.(key)
+  end
+
   def expand_root_keys(value, _fun), do: expand_fields(value, &to_string/1)
 
   defp maybe_expand_fields(values, fun) when is_list(values) do

test/jsonapi/serializer_test.exs

@@ -667,12 +667,15 @@ defmodule JSONAPI.SerializerTest do
       encoded = Serializer.serialize(PostView, data, nil)
 
       attributes = encoded[:data][:attributes]
+      relationships = encoded[:data][:relationships]
       included = encoded[:included]
 
       assert attributes["full-description"] == data[:full_description]
       assert attributes["body"]["data_attr"] == "foo"
       assert attributes["inserted-at"] == data[:inserted_at]
 
+      assert Map.has_key?(relationships, "best-comments")
+
       author = Enum.find(included, &(&1[:type] == "user" && &1[:id] == "2"))
       assert author != nil
       assert author[:attributes]["last-name"] == "bonds"