Cherrypick from opengever.core release 4.0.

Use correct content-type when returning 'OK' responses.

Conflicts:
	opengever/base/utils.py
	opengever/task/browser/accept/utils.py
Updated the changelog.
Merge pull request #528 from 4teamwork/pg_deadline_modifier_fix

Set content-type correctly for ok_response in inter-client functionalities.