Fix 0 param interpolation

There is a little bug in the parameter interpolation, when we want to use `0` as parameter:
E.g:
```
var resource = httpi.resource( "api/friends/:id" );
var promise = resource.get({
    params: {
        id: 0
    }
});
```
In this case, the interpolation gives incorrect result (`api/friends`), because `0` in JS is falsy.
It is not really common, to use zero as parameter, but it happens once in a while.