TimeEntries spent_on date display fix.

The detail view of logged time entries does not show the logged date
(spent_on), but instead displays the current time.

Using the date directive as an element tag with date-value set
seems to fix the issue.

The following example contains a time entry with ``spent_on`` set to 2014-11-02:

![Edit date correct](https://dl.dropboxusercontent.com/u/270758/op_spent_on_fe.png)

yet, the detail view shows the current date.

![Displays incorrectly](https://dl.dropboxusercontent.com/u/270758/op_spent_on_f.png)

I've also added this as a work package at https://community.openproject.org/work_packages/17222