Emit Content-Length: 0 for empty PUTs

HttpUrlConnection reverts Content-Length: 0 on PUT without
doOutput(true), similar to POST.  This commit allows Azure container
create to succeed.  Closes #1420.
Merge pull request #1493 from maginatics/http-put-zero-content-length-1.6.x

Emit Content-Length: 0 for empty PUTs