support adding default meta data
Also new settings: MINIO_STORAGE_MEDIA_OBJECT_METADATA and
MINIO_STORAGE_STATIC_OBJECT_METADATA
example:
py
MINIO_STORAGE_MEDIA_OBJECT_METADATA = {"Cache-Control": "max-age=1000"}
fix issue with directory listing names
Minio has changed in the last months to be more picky about path names so we
now enure that we don't create path prefixes with a // suffix.