Fix Active Support test warnings

The namespace ivar needs a definition check to not warn on Ruby 2.7

Author: skipkayhil

activesupport/test/cache/behaviors/cache_logging_behavior.rb

@@ -77,7 +77,7 @@ def assert_logs(pattern, &block)
       assert_match pattern, io.string
     end
 
-    def key_pattern(key, namespace: @namespace)
+    def key_pattern(key, namespace: defined?(@namespace) && @namespace)
       /#{Regexp.escape namespace.to_s}#{":" if namespace}#{Regexp.escape key}/
     end
 end