api: Set content-encoding properly if set. (#2245)

Additionally don't set content-type if not present, golang http
server automaticaly handles this and sets it automatically.

xl-v1-list-objects.go

@@ -97,6 +97,7 @@ func (xl xlObjects) listObjects(bucket, prefix, marker, delimiter string, maxKey
 			Name:    objInfo.Name,
 			ModTime: objInfo.ModTime,
 			Size:    objInfo.Size,
+			MD5Sum:  objInfo.MD5Sum,
 			IsDir:   false,
 		})
 	}