Fix query parameter passing

Author: twistedfall

src/client.rs

@@ -218,7 +218,12 @@ impl Client {
 
         if let Some(query) = endpoint.query() {
             let query_string = serde_qs::to_string(&query).expect("serialize the query correctly");
-            url.push_str(&query_string);
+            if !query_string.is_empty() {
+                if !url.ends_with('?') {
+                    url.push('?');
+                }
+                url.push_str(&query_string);
+            }
         }
 
         let mut request = self.client.request(endpoint.method(), url);