Uploading empty artifact S3 causes 501

[jonasfj]

credits @bclary for reporting it.

For some reason the error from my integration test was suppressed initially, that's fixed now...
Which means we can reproduce it...

[jonasfj]

Empty artifacts is broken, because contentLength zero is the default value... so golang/http assumes you didn't set contentLength uses transfer-encoding: chunked, which S3 doesn't support.

Fun bug which probably requires a work-around: golang/go#20257

Hmm, maybe I can patch my fake S3 implementation to check a content-length is present :)

[jonasfj]

Fix added to #373

[jonasfj]

Fixed