support for multivalued parameters in get/post requests

Currently cljs-http does not support sending multivalued parameters (i.e. array values) in POST/GET requests
That means I can't do this: GET http://example.com?my-repeated-val=1&my-repeated-val=2&my-repeated-val=3

This PR fixes that.

(By default, in the server, Ring's wrap-params middleware would transform the above request into {"my-repeated-val" [1 2 3]})