Do not set Last-Modified header in initial stored, lazy rendition "empty" response

Prior to the fix for NXP-27337 in Nuxeo 10.10-HF15, the initial response for a stored, lazy, Folderish rendition included the following headers and did NOT include a Last-Modified header:

Content-Disposition: inline; filename=inprogress
Content-Type: text/plain; empty=true
Content-Length: 0

The fix for NXP-27337 introduced a Last-Modified header into the initial response which differs from the Apache Chemistry CMIS Reference Implementation and confuses some CMIS clients.

The pre-10.10 HF15 functionality regarding the Last-Modified response header in this scenario should be restored.