fix when cache file size big than 32*1024

[zonghuili2019]

when cache the file， if file size big than 32*1024，there is a bug in the function cachedWriter:Write. the copyBuffer function in the io.go file will call Write function multile times。 go version is 1.14.1,gin1.7.1, the way is gin.Default().Static("/static", "./static").