azureblob,memory,pcloud: fix setting of mime types

Before this change the backend was reading the mime type of the
destination object instead of the source object when uploading.

This changes fixes the problem and introduces an integration test for
it.

See: https://forum.rclone.org/t/is-there-a-way-to-get-rclone-copy-to-preserve-metadata/20682/2

backend/memory/memory.go

@@ -592,7 +592,7 @@ func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, op
 		data:     data,
 		hash:     "",
 		modTime:  src.ModTime(ctx),
-		mimeType: fs.MimeType(ctx, o),
+		mimeType: fs.MimeType(ctx, src),
 	}
 	buckets.updateObjectData(bucket, bucketPath, o.od)
 	return nil