Correcting the normalization of chebwin window function

I was trying to work on https://github.com/scipy/scipy/issues/2221 but noticed that the current normalization for Chebwin window function works correctly only for significant attenuation : 
##### Current :
```
In [3]: chebwin(6, 10)
Out[3]: 
array([ 1.46877571,  0.89172335,  1.        ,  1.        ,  0.89172335,
        1.46877571])

In [4]: chebwin(6, 20)
Out[4]: 
array([ 0.54057352,  0.77676753,  1.        ,  1.        ,  0.77676753,
        0.54057352])
```
In the case of 10dB attenuation, chebwin is not correctly normalized. The tests missed this because they were on 40dB(significant) attenuation.

##### This PR : 
```
In [2]: chebwin(6, 10)
Out[2]: array([ 1.        ,  0.60712017,  0.68083915,  0.68083915,  0.60712017,  1.        ])

In [3]: chebwin(6, 20)
Out[3]: 
array([ 0.54057352,  0.77676753,  1.        ,  1.        ,  0.77676753,
        0.54057352])
```

The results are in tune with Octave and Matlab.

##### Octave : 
```
octave:8> chebwin(6, 10)
ans =

   1.00000
   0.60712
   0.68084
   0.68084
   0.60712
   1.00000

octave:9> chebwin(6, 20)
ans =

   0.54057
   0.77677
   1.00000
   1.00000
   0.77677
   0.54057

```
##### Matlab : 
```
>> chebwin(6, 10)

ans =

    1.0000
    0.6071
    0.6808
    0.6808
    0.6071
    1.0000

>> chebwin(6, 20)

ans =

    0.5406
    0.7768
    1.0000
    1.0000
    0.7768
    0.5406
```
The previous tests missed this because it did not cover a low attenuation case.