tigrisfs mounts bucket as read-write even when access key is read-only

[lillian-b]

I am able to mount a bucket to which I have read-only access as read-write, and change files on the local disk. tigrisfs prints a warning about a 403 while flushing the file, but tries again anyway.

[ovaistariq]

Thanks for the report. In this case we should validate the permission of the keys before allowing it to be mounted rw.