skip isFileUnchanged checking since ttl always change

skip checking since ttl always change. Fixing
https://github.com/chrislusf/seaweedfs/issues/166

go/storage/volume.go

@@ -152,6 +152,9 @@ func (v *Volume) NeedToReplicate() bool {
 // isFileUnchanged checks whether this needle to write is same as last one.
 // It requires serialized access in the same volume.
 func (v *Volume) isFileUnchanged(n *Needle) bool {
+	if v.Ttl == EMPTY_TTL || v.Ttl.String() == "" {
+		return true
+	}
 	nv, ok := v.nm.Get(n.Id)
 	if ok && nv.Offset > 0 {
 		oldNeedle := new(Needle)