memory: fix dst mutating src after server-side copy

Before this change, the Memory backend's Copy method created a dst object that
referenced the src's objectData by pointer instead of making a copy. While this
minimized memory usage, an unintended consequence was that subsequently mutating
the src (such as changing the modtime) would inadvertently also mutate the dst,
and vice versa.

This change fixes the issue and adds a test.

backend/memory/memory.go

@@ -482,7 +482,8 @@ func (f *Fs) Copy(ctx context.Context, src fs.Object, remote string) (fs.Object,
 	if od == nil {
 		return nil, fs.ErrorObjectNotFound
 	}
-	buckets.updateObjectData(dstBucket, dstPath, od)
+	odCopy := *od
+	buckets.updateObjectData(dstBucket, dstPath, &odCopy)
 	return f.NewObject(ctx, remote)
 }