Do not log Download event for CMIS getContentStream HEAD request

Nuxeo 7.3+ introduces Download audit events. Some repositories receives hundreds of CMIS getContentStream HEAD requests from various CMIS clients every minute. These HEAD requests are followed by GET requests only when the ETag for the content has changed.

Nuxeo already generates a LoginSuccess audit event for every one of these HEAD requests. This is not a major problem since these events are not Document-related. OTOH, it is a major problem for each of these HEAD requests to litter Document History with numerous, "end-user-irrelevant" Download events.