Fix object copy with empty source

When s3 client make a copy request with empty copy source,
current swift3 will return a 500 error. This is caused from
a lack of header validation.

This fixes the behavior to make a 400 Bad Request (InvalidArgument).

Change-Id: I60c7e480582f5b9258433306bbf228b7291e6236
Merge "Fix object copy with empty source"