copyBuffer: Implement copyBuffer to support old Golang versions.
copyBuffer: Implement copyBuffer to support old Golang versions.

Golang versions < 1.5.x do not implement io.CopyBuffer, all
dependent applications will fail if they wish to build their
binaries for linux distributions which are still shipping older
versions of Golang.

minio/minio-go,abperiasamy/minio-go,harshavardhana/minio-go,awwalker/minio-go,donatello/minio-go