Support use ';' as separator of "Cache-Control"

Picasso set "Cache-Control" with this line of code
```
connection.setRequestProperty("Cache-Control", "only-if-cached;max-age=" + Integer.MAX_VALUE);
```

But okhttp only recognize ',' as separator of directives. I have test the behavior of HttpUrlConnection, it don't support ';' either.
 I'm not sure should okhttp add this compatibility.