Fixes #625 Allow head/body to be null
Fixes #625 Allow head/body to be null

fzzzy/pageshot,mozilla-services/pageshot,mozilla-services/screenshots,mozilla-services/pageshot,mozilla-services/pageshot,mozilla-services/screenshots,fzzzy/pageshot,mozilla-services/pageshot,mozilla-services/screenshots,mozilla-services/screenshots,fzzzy/pageshot,fzzzy/pageshot