S3FS: correct verifying of buckets when IAM policy is applied.
S3FS: correct verifying of buckets when IAM policy is applied.

This fixes Issue 120.

piranna/pyfilesystem,pombreda/pyfilesystem,flying-circus/pyfilesystem,zopyx/pyfilesystem,PyFilesystem/pyfilesystem,pmitros/pyfs,pmitros/pyfs,Konubinix/pyfilesystem