基类InvokeNextBefore和InvokeNextAfter方法改为virtual方法，这样子类不必重写所有两个方法

Falcon.Middleware.Test/MiddlewareTest.cs

@@ -10,10 +10,17 @@ namespace Falcon.Middleware.Test
         public MiddlewareTest(RequestDelegate next) : base(next) {
         }
 
+        /// <summary>
+        /// 在调用next之后执行
+        /// </summary>
+        /// <param name="context">请求上下文</param>
         public async override void InvokeNextAfter(HttpContext context) {
             await context.Response.WriteAsync("\nInvokeNextAfter");
         }
-
+        /// <summary>
+        /// 在调用next之前执行
+        /// </summary>
+        /// <param name="context">请求上下文</param>
         public async override void InvokeNextBefore(HttpContext context) {
             await context.Response.WriteAsync("InvokeNextBefore\n");
         }

Falcon.Middleware/MiddlewareBase.cs

@@ -31,11 +31,11 @@ namespace Falcon.Middleware
         /// 在执行下一个中间件之前执行
         /// </summary>
         /// <param name="context">请求上下文</param>
-        public abstract void InvokeNextBefore(HttpContext context);
+        public virtual void InvokeNextBefore(HttpContext context) { }
         /// <summary>
         /// 在执行下一个中间件之后执行
         /// </summary>
         /// <param name="context">请求上下文</param>
-        public abstract void InvokeNextAfter(HttpContext context);
+        public virtual void InvokeNextAfter(HttpContext context) { }
     }
 }