Updating logic to successfully detect empty request body
Add MultiPrefer type and action to handle multiple Prefer header requests

Resolves: https://www.pivotaltracker.com/story/show/81242476
Merge pull request #609 from fcrepo4/repeated-put-conflict

Updating logic to successfully detect empty request body
Add support to the web UI to trigger the file picker when no file attached for datastream creation.

Resolves: https://www.pivotaltracker.com/story/show/78847348