Add ability to use Standard Cookie Spec.

[mplubratt]

I'm getting an HttpClient warning "Invalid expires attribute". I found the following post in stackOverflow that describes the problem and solution.

https://stackoverflow.com/questions/36473478/fixing-httpclient-warning-invalid-expires-attribute-using-fluent-api/40697322

I'm not familiar enough with Clojure and clj-http to implement a solution. Thank you!

[rymndhng]

@mplubratt Did you intend on using cookies? If not, you may be able to ignore the warnings by disabling the cookie policy with :cookie-policy :none

For a similar issue, see https://github.com/dakrone/clj-http/issues/578.