Apt::source should pass release to apt::pin

In case pinning is used in apt::source declaration, the 'release' is not sent as parameter to apt::pin, which cause faulty pinning file like : 

```
# wheezy-backports
Explanation: : wheezy-backports
Package: *
Pin: origin "ftp.fr.debian.org"
Pin-Priority: -10
```

which pin all debian repositories, instead it should be : 

```
# wheezy-backports
Explanation: : wheezy-backports
Package: *
Pin: release a=wheezy-backports
Pin-Priority: -10
``` 

This pull request fix, I think, this issue, and also update spec to ensure release is correctly passed.