[#185] Fix payload reader

When we use io.CopyBuffer it check for exact io.EOF matching,
so we need keep original EOF error otherwise io.CopyBuffer returns error

Signed-off-by: Denis Kirillov <d.kirillov@yadro.com>

internal/frostfs/frostfs.go

@@ -263,6 +263,9 @@ type payloadReader struct {
 
 func (x payloadReader) Read(p []byte) (int, error) {
 	n, err := x.ReadCloser.Read(p)
+	if err != nil && errors.Is(err, io.EOF) {
+		return n, err
+	}
 	return n, handleObjectError("read payload", err)
 }