Allow plugging custom body deserializers when deserializing a browser response body

Today there's no way to use the custom serializers when deserializing a browser response body. This PR changes that. 

It includes one breaking change; the `DeserializeJson` overload which took a `JavaScriptSerializer` has been removed, please tell if you want it added back in again.

Also, why is content type passed to `IBodyDeserializer.Deserialize` when it isn't used in any of the deserializers?