Allow undefined values in expires assertion

Using server.route.config.cache.expiresIn = 0 in hapi causes the following error: "Error: Rule must include one of expiresIn or expiresAt but not both {"expiresIn":0}". 

The call to Catbox.Policy() here https://github.com/spumko/hapi/blob/master/lib/route.js#L84 passes an undefined expiresAt attribute in hapi's route.js code. My first thought was to check for undefined before calling Catbox.Policy() in route.js but I found that Catbox.Policy is similarly called with both expiresIn and expiresAt in hapi's pack.js so maybe there's some kind of convention that I'm not sure about. 

Of course if you trigger the code in pack.js by running "var cache = server.cache('countries', { expiresIn: 0 });" the error is not generated because of the truthy condition (https://github.com/spumko/hapi/blob/master/lib/pack.js#L669) that sets the settings parameter that ends up not setting any attribute.