request: test adding duplicate headers to the request (#519)

src/async_impl/request.rs

@@ -642,6 +642,27 @@ mod tests {
         assert_eq!(r.headers, headers);
     }
 
+    #[test]
+    fn add_headers_multi() {
+        let client = Client::new().unwrap();
+        let some_url = "https://google.com/";
+        let mut r = client.post(some_url).unwrap();
+
+        let header = Host {
+            hostname: "google.com".to_string(),
+            port: None,
+        };
+
+        let mut headers = Headers::new();
+        headers.set(header);
+
+        // Add a copy of the headers to the request builder
+        let r = r.headers(headers.clone()).build();
+
+        // then make sure they were added correctly
+        assert_eq!(r.headers, headers);
+    }
+
     #[test]
     fn add_body() {
         let client = Client::new().unwrap();