commit instead of flush on issue assign update
    
    Like it does earlier in the file, when resetting the assignee. If we
    don't commit the transaction, SA won't let us access the updated
    attributes, which is why we were (correctly) getting None in the reset
    case (since it calls commit), but we were getting the old assignee in
    the reassign case.
    
    This fixes #1896.
    
    Signed-off-by: Ricky Elrod <ricky@elrod.me>