make cache package to work across multi handlers, remove the old 'WrapHandler' and keep the cache.Handler as documented only

Former-commit-id: b030cd92d26a9f646b060e379b3702b9a677749b

_examples/file-server/single-page-application/basic/main_test.go

@@ -3,6 +3,7 @@ package main
 import (
 	"io/ioutil"
 	"path/filepath"
+	"runtime"
 	"strings"
 	"testing"
 
@@ -35,6 +36,7 @@ func (r resource) loadFromBase(dir string) string {
 	}
 
 	result := string(b)
+
 	return result
 }