Last modified date for Swift3 middleware isn't formatted correctly

"s3cmd ls" fails against swift's S3 interface with
  An unexpected error has occurred.
  Please report the following lines to:
   <email address hidden>

Problem: ValueError: time data '2011-08-30T13:37:32.852790' does not match format '%Y-%m-%dT%H:%M:%S.000Z'

Indeed, BucketController.GET in swift3.py preparing the ListBucketResult response returns the LastModified tag with the date in native format, not the expected xxxxxZ format.

I suggest to simply add a Z to the date, in swift3.py the corresponding line in BucketController.GET would then read

"".join(['<Contents><Key>%s</Key><LastModified>%sZ</LastModif'\

instead of

"".join(['<Contents><Key>%s</Key><LastModified>%s</LastModif'\