some major improvements to the (server-side) cache middleware and an example of a client-side responsibility cache

Former-commit-id: 93d3a7a6f163c6d49f315f86d10e63f7b1b1d93a

_examples/cache/client-side/main.go

@@ -0,0 +1,27 @@
+// Package main shows how you can use the `WriteWithExpiration`
+// based on the "modtime", if it's newer than the request header then
+// it will refresh the contents, otherwise will let the client (99.9% the browser)
+// to handle the cache mechanism, it's faster than iris.Cache because server-side
+// has nothing to do and no need to store the responses in the memory.
+package main
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/kataras/iris"
+)
+
+var modtime = time.Now()
+
+func greet(ctx iris.Context) {
+	ctx.Header("X-Custom", "my  custom header")
+	response := fmt.Sprintf("Hello World! %s", time.Now())
+	ctx.WriteWithExpiration([]byte(response), modtime)
+}
+
+func main() {
+	app := iris.New()
+	app.Get("/", greet)
+	app.Run(iris.Addr(":8080"))
+}