api: fix legacy /post/index and /tag/index endpoints.

Fixup for a1cd9d2b5. The route order matters here, the legacy endpoints
need to go first.

test/functional/legacy_controller_test.rb

@@ -0,0 +1,25 @@
+require 'test_helper'
+
+class LegacyControllerTest < ActionDispatch::IntegrationTest
+  context "The legacy controller" do
+    context "post action" do
+      should "work" do
+        get "/post/index.xml"
+        assert_response :success
+
+        get "/post/index.json"
+        assert_response :success
+      end
+    end
+
+    context "tag action" do
+      should "work" do
+        get "/tag/index.xml"
+        assert_response :success
+
+        get "/tag/index.json"
+        assert_response :success
+      end
+    end
+  end
+end