[tracker] Filters aren't cleared correctly

Filters are not always cleared correctly

Example 1:
1) Go to http://tracker.xubuntu.org/#tab-details/s=todo,inprogress,blocked+sort=assignee+sortdir=asc
2) Move to the Overview tab
3) Move back to the Details tab
Result:
The status filter says "Multiple..." even if there is actually no status filter set

Example 2:
1) Go to the Details tab and apply an assignee filter
2) Go to the Overview tab and click on a specification link
Result:
Two filters are applied; assignee and specification - only specification should be filtered