Set Content-Type of “application/octet-stream” for continuous _changes

This prevents clients and gateways from trying to sniff a content-type
by buffering up the beginning of the response, which can cause the
actual client app code from receiving data.
Clearer logging when importing existing bucket docs into database.
Better error handling writing multipart documents (prevents a crash)

Fixes #230
Fix “composite literal uses unkeyed fields” warnings in debug.go