Fix "ArgumentError: Invalid response name: unprocessable_entity" with Rack 3.1.0+ in assert_response (#39889).

Rack 3.1.0 changed the symbol for HTTP status code 422 from `:unprocessable_entity` to `:unprocessable_content`.
Due to the change, `assert_response(:unprocessable_entity, ...)` raises ArgumentError with Rack 3.1.0+.

This fix is a follow-up to r22837.


git-svn-id: https://svn.redmine.org/redmine/trunk@22876 e93f8b46-1217-0410-a6f0-8f06a7374b81

test/integration/api_test/versions_test.rb

@@ -120,7 +120,7 @@ class Redmine::ApiTest::VersionsTest < Redmine::ApiTest::Base
         :params => {:version => {:name => ''}},
         :headers => credentials('jsmith'))
     end
-    assert_response :unprocessable_entity
+    assert_response :unprocessable_content
     assert_select 'errors error', :text => "Name cannot be blank"
   end