Specify sub second precision for ISO 8601 dates

This is needed for jets3t (v0.7.2) to work with fakes3, due to some (perhaps) overly strict date parsing on their side. Hopefully this shouldn’t affect interaction with any other clients, the tests are passing at least.

Should also be mentioned that the examples over [here](http://docs.aws.amazon.com/AmazonS3/latest/API/RESTBucketGET.html) (i.e. the official AWS S3 API documentation) specifies a precision of three (3) digits.