Description of problem:
The property of test run failed to be deleted at the first time after edit it's value.

Version-Release number of selected component (if applicable):
tcms-3.5.0-4

How reproducible:
100%

Steps to Reproduce:
1.Enter into a plan.
2.Click "Runs" label.
3.Choose a run and click it's id to open the run.
4.Click the "Edit this value" label behind the property you want to edit in environment option.(if no property exists, pls add one firstly)
5.Choose a new verision in the combo box.
6.Click "Submit" button.
7.Click the "Remove this property" label behind the "Edit this value" label of the property and click OK in the confirmation dialog
  
Actual results:
After above step 7, the property still exists, that is, failed to be removed.

Expected results:
After above step 7, the property should be deleted.

Additional info:
After above step 7, if repeat the step 7, that is, try to remove it once again, the property can be deleted at this time.