Fixed an issue in req.filter function

Hi Chris,

First of all thanks for the restify-validator module. It saved me a lot of time.
I have fixed an issue in req.filter function. When I pass param with dot notation it was not sanitizing. For example if there is a param like this

    req.params.address.city

then I couldn't sanitize the city value like this

    req.sanitize('address.city').trim();

I made a small change in the code to solve this problem. Please take time to merge this into the project.

Cheers,
Khaja Naquiuddin.