fix the mount.mounted opts default value behavior

the string "defaults" is useful when mounting a volume but doesn't show
up in the options of already-mounted volumes, so filter it out when
determining whether options have changed and we should therefore
remount.

my volume was remounting every time until I made this fix, simply because I wasn't specifying any options myself.
Since the argument default you are passing in is not a list won't this iterate over the string 'defaults'?