fix span_range for week frame

I found the problem in `span_range` function related to week frame. 

```
In [5]: arrow.Arrow.span_range('week', arrow.Arrow(2014, 9, 10), arrow.Arrow(2014, 9, 16))
Out[5]: 
[(<Arrow [2014-09-08T00:00:00+00:00]>,
  <Arrow [2014-09-14T23:59:59.999999+00:00]>)]
```
but if I shift start_date early, I get the extra week:
```
In [6]: arrow.Arrow.span_range('week', arrow.Arrow(2014, 9, 9), arrow.Arrow(2014, 9, 16))
Out[6]: 
[(<Arrow [2014-09-08T00:00:00+00:00]>,
  <Arrow [2014-09-14T23:59:59.999999+00:00]>),
 (<Arrow [2014-09-15T00:00:00+00:00]>,
  <Arrow [2014-09-21T23:59:59.999999+00:00]>)]
```
The second behavior, I think, correct: all tests of  other time frames include period with end date. So I correct the `span_range` test for week (add absent period) and fix the behavior of `span_range` to pass it.