Make route middleware/handler mockable

[wxiaoguang]

To mock a handler:

web.RouteMock(web.MockAfterMiddlewares, func(ctx *context.Context) {
	// ...
})
defer web.RouteMockReset()

It helps:

• Test the middleware's behavior (assert the ctx.Data, etc)
• Mock the middleware's behavior (prepare some context data for handler)
• Mock the handler's response for some test cases, especially for some integration tests and e2e tests.