Due date bug

Please review this fix for a due_date bug.

Also includes a bit of refactoring for readability.

Actually, the fix was kinda weird - refactored the code fixed it.

From:
```ruby
      if issue.due_date = row[@attrs_map["due_date"]].blank?
        nil
      else Date.parse(row[@attrs_map["due_date"]])
      end
```
To:
```ruby
      issue.due_date = if row[@attrs_map["due_date"]].blank?
                         nil
                       else
                         Date.parse(row[@attrs_map["due_date"]])
                       end
```