Fix missing Location header in 0.9.x (#1362)

* Add failing test to demonstrate regression

It looks like this test was originally added as part of the Location
header work, but was subsequently removed.

See: 933750c

* Fix location header to be set correctly, fixes #1361

Previously, a change had been make to use string keys instead of symbol keys, see
010f024

The change did not update how the location header was set to also use string keys.

This PR resolves the issue by also using a string key.

lib/jsonapi/acts_as_resource_controller.rb

@@ -234,7 +234,7 @@ def render_results(operation_results)
         render_options[:body] = JSON.generate(content)
       end
 
-      render_options[:location] = content[:data]["links"][:self] if (
+      render_options[:location] = content[:data]["links"]["self"] if (
         response_doc.status == :created && content[:data].class != Array && content[:data]["links"]
       )
 

test/controllers/controller_test.rb

@@ -591,6 +591,7 @@ def test_create_simple
     assert_equal 'JR is Great', json_response['data']['attributes']['title']
     assert_equal 'JSONAPIResources is the greatest thing since unsliced bread.', json_response['data']['attributes']['body']
     assert_equal "http://test.host/posts/#{json_response['data']['id']}", json_response['data']['links']['self']
+    assert_equal json_response['data']['links']['self'], response.location
   end
 
   def test_create_simple_id_not_allowed