When content-lenght is less than content-memory-threshold the copy operation is very slow in clammit

[zhaidarbek]

For example with content-memory-threshold=100mb and file size equal to 50mb the io.Copy(mb, input) in multireader.go takes about 10 minutes when running clammit inside docker container. If i lower content-memory-threshold to 10mb it takes about 30 seconds.