Remove unnecessary header field in ErrorResponse

[vadmeste]

ErrorResponse.Headers is causing a panic inside golang http package.

The following code can be easily crashed if GetObject API returns an error,
such as ErrSlowDown:

  reader, err := s3Client.GetObject("my-bucketname", "my-source-objectname", minio.GetObjectOptions{})
  if err != nil {
    log.Fatalln(err)
  }
  defer reader.Close()

  stat, err := reader.Stat()
  if err != nil {
    log.Fatalln(err)
  }

  n, err := s3Client.PutObject("my-bucketname", "my-target-objectname", reader, stat.Size, minio.PutObjectOptions{})
  if err != nil {
    log.Fatalln(err)
  }

The reason is that reader is passed to s3Client.PutObject therefore to golang http
via request.Body. Since, reader.Read() can return an ErrorResponse, that error will
be tested by golang http, but since ErrorResponse is uncomparable, then it will cause
a crash with the following stackstrace:

  panic: runtime error: comparing uncomparable type minio.ErrorResponse

  goroutine 20 [running]:
  net/http.(*Request).write(0xc0002c2300, 0x761e60, 0xc000069f80, 0x0, 0x0, 0x0, 0x7628a0, 0xc000518780)
          /home/vadmeste/work/go/src/net/http/request.go:647 +0x74c
  net/http.(*persistConn).writeLoop(0xc0000a17a0)
          /home/vadmeste/work/go/src/net/http/transport.go:1888 +0x1b8
  created by net/http.(*Transport).dialConn
          /home/vadmeste/work/go/src/net/http/transport.go:1339 +0x966
  exit status 2

Hence, removing Headers since it is a map and it is uncomparable.

This will fix minio/mc#2327

[harshavardhana]

FWIW this is a Go implementation bug, but this workaround is perfect for now.

LGTM tested.