Allow null in useDb to unset connection on page

Author: mythz

src/ServiceStack.Redis/RedisScripts.cs

@@ -60,7 +60,11 @@ T exec<T>(Func<IRedisClient, T> fn, ScriptScopeContext scope, object options)
 
         public IgnoreResult useRedis(ScriptScopeContext scope, string redisConnection)
         {
-            scope.PageResult.Args[RedisConnection] = redisConnection;
+            if (redisConnection == null)
+                scope.PageResult.Args.Remove(RedisConnection);
+            else
+                scope.PageResult.Args[RedisConnection] = redisConnection;
+
             return IgnoreResult.Value;
         }