Use url.format to rebuild query params on to endpointUrl

Fixes #44

This will use url.format to rebuild query params on to endpointUrl so that we can properly proxy requests that have multiple query params of the same name.

eg.

http://my.apy.com/resource?foo=bar&foo=baz 

The above query string was previously being proxied to the endPoint as ?foo=bar,baz