Fix constant reference

Update the long key handling test so it triggers truncation in the Redis cache store.

activesupport/lib/active_support/cache/redis_cache_store.rb

@@ -368,7 +368,7 @@ def normalize_key(key, options)
 
         def truncate_key(key)
           if key.bytesize > max_key_bytesize
-            suffix = ":sha2:#{Digest::SHA2.hexdigest(key)}"
+            suffix = ":sha2:#{::Digest::SHA2.hexdigest(key)}"
             truncate_at = max_key_bytesize - suffix.bytesize
             "#{key.byteslice(0, truncate_at)}#{suffix}"
           else

activesupport/test/cache/behaviors/cache_store_behavior.rb

@@ -309,8 +309,7 @@ def test_crazy_key_characters
   end
 
   def test_really_long_keys
-    key = "".dup
-    900.times { key << "x" }
+    key = "x" * 2048
     assert @cache.write(key, "bar")
     assert_equal "bar", @cache.read(key)
     assert_equal "bar", @cache.fetch(key)