Add reason for version fetching and invalidation

Author: mariusvniekerk

s3fs/core.py

@@ -928,6 +928,9 @@ def __init__(self, s3, path, mode='rb', block_size=5 * 2 ** 20, acl="",
                 self.size = self.details['size']
             elif self.fs.version_aware:
                 self.version_id = self.details.get('VersionId')
+                # In this case we have not managed to get the VersionId out of details and 
+                # we should invalidate the cache and perform a full head_object since it 
+                # has likely been partially populated by ls.
                 if self.version_id is None:
                     self.fs.invalidate_cache(self.path)
                     self.details = self.fs.info(self.path)