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

Right now the `modify_deadline` form does not work with the `plonetheme.teamraum` because of a wrong request content-type (see [RemoteDeadlineModifier](https://github.com/4teamwork/opengever.core/blob/fa8dff2032e2624b8ad6c2cf97f2f8e38ae3db3a/opengever/task/browser/modify_deadline.py#L99). This problem is already solved in the task feature branch (Release 4.0). Because there are no other changes, I cherry-picked the fix for the master, so we can fix this problem with the next bugfix-release.

Following a simple example how it look's the prefix manager with deleted objects:
![bildschirmfoto 2014-09-04 um 08 45 41](https://cloud.githubusercontent.com/assets/485755/4145764/5093dee2-33ff-11e4-9792-2a4c6a531793.png)


@lukasgraf please have a look ...