redefinition of function get_closed_date in task.py

In latest trunk (revision 299.1.12):
$pyflakes GTG/core/task.py
task.py:225: redefinition of function 'get_closed_date' from line 84

That is because:
task.py, line 84:

    def get_closed_date(self):
        return self.closed_date

was added by bertrand (246.1.10) , while

task.py, line 225:
    def get_closed_date(self):
        if self.closed_date:
            return str(self.closed_date)
        else:
            return ''"
was added by bertrand (302) and ploum (40.1.21, 117.1.1)

The second one is more conservative, so here's the patch to remove the first one.