CMIS: include the ETag in the initial 302 response header when using s3 direct download

Client uses HEAD requests or IF-NONE-MATCH GET requests with CMIS content URLs to regularly poll for document changes.

When S3 directdownload is enabled, it appears two requests are required for each poll since the response from the first request is a 302 redirect that must be followed by a subsequent request.

Nuxeo has the document ETag available when it generates the initial 302 response – the response should include this ETag to allow this scenario to work with only one request/no redirect