s3a to set fake directory marker contentType to application/x-directory

S3AFileSystem doesn't set a contentType for fake directory files, causing it to be inferred as "application/octet-stream". But fake directory files created through the S3 web console have content type "application/x-directory". We may want to adopt the web console behavior as a standard, since some systems will rely on content type and not size + trailing slash to determine if an object represents a directory.