Make empty params to request give no query string

The previous behaviour was to insert an empty query string
for an empty map of params, but leave out the key in case it was nil.
This change makes it consistently leave out the :query-string in both cases.

The previous behaviour caused problems when using the ring request/request-url
function - it would (and still does as of time of writing) add a trailing ?
to the url when the empty query string is there. This can be confusing.

Discussion: ring-clojure/ring#543 (comment)

Author: rutchkiwi

src/ring/mock/request.clj

@@ -47,7 +47,7 @@
 
 (defn- combined-query [request params]
   (let [query (:query-string request)]
-    (when (or query params)
+    (when (or query (not-empty params))
       (string/join "&"
         (remove string/blank?
                 [query (encode-params params)])))))

test/ring/mock/request_test.clj

@@ -127,7 +127,12 @@
     (is (= (-> {}
                (query-string {:a "b"})
                (query-string {:c "d"}))
-           {:query-string "c=d"}))))
+           {:query-string "c=d"})))
+  (testing "empty params is the same as nil, e.g no query string"
+    (is (= (:query-string (request :get "/" nil))
+           (:query-string (request :get "/" {}))
+           nil))))
+
 
 (deftest test-body
   (testing "string body"