Fix Accept.best_matches when q=0

Only includes Accept items in consideration for best_matches if they have q>0, as a quality of 0 should be equivalent to not being included in the list at all. 

This agrees with RFC2616 (http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.9): 
```
If a parameter has a quality value of 0, then content with this parameter is `not acceptable' for the client.
```