Add JSON support for GET /my/pins with no pagination and tests.

test/controllers/my/pins_controller_test.rb

@@ -11,4 +11,14 @@ class My::PinsControllerTest < ActionDispatch::IntegrationTest
     assert_response :success
     assert_select "div", text: /#{users(:kevin).pins.first.card.title}/
   end
+
+  test "index as JSON" do
+    expected_ids = users(:kevin).pins.ordered.pluck(:card_id)
+
+    get my_pins_path(format: :json)
+
+    assert_response :success
+    assert_equal expected_ids.count, @response.parsed_body.count
+    assert_equal expected_ids, @response.parsed_body.map { |card| card["id"] }
+  end
 end