Add UTF8 to server-redis HTML

frameworks/Crystal/kemal/server-redis.cr

@@ -11,7 +11,7 @@ class CONTENT
   UTF8 = "; charset=UTF-8"
   JSON = "application/json"
   PLAIN = "text/plain"
-  HTML = "text/html"
+  HTML = "text/html" + UTF8
 end
 
 ID_MAXIMUM = 10_000