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.
no, see line 90. the function is documented as taking a string or list of strings, so I changed the default value because it's a no-op change that reduces the number of types the function has to consider.