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

src/request.rs

@@ -580,7 +580,7 @@ fn fmt_request_fields<'a, 'b>(f: &'a mut fmt::DebugStruct<'a, 'b>, req: &Request
 #[cfg(test)]
 mod tests {
     use {body, Client, Method};
-    use header::{HOST, HeaderMap, HeaderValue, CONTENT_TYPE};
+    use header::{ACCEPT, HOST, HeaderMap, HeaderValue, CONTENT_TYPE};
     use std::collections::{BTreeMap, HashMap};
     use serde_json;
     use serde_urlencoded;
@@ -678,6 +678,30 @@ mod tests {
         assert_eq!(r.headers(), &headers);
     }
 
+    #[test]
+    fn add_headers_multi() {
+        let client = Client::new();
+        let some_url = "https://google.com/";
+        let r = client.post(some_url);
+
+        let header_json = HeaderValue::from_static("application/json");
+        let header_xml = HeaderValue::from_static("application/xml");
+
+        let mut headers = HeaderMap::new();
+        headers.append(ACCEPT, header_json);
+        headers.append(ACCEPT, header_xml);
+
+        // Add a copy of the headers to the request builder
+        let r = r.headers(headers.clone()).build().unwrap();
+
+        // then make sure they were added correctly
+        assert_eq!(r.headers(), &headers);
+        let mut all_values = r.headers().get_all(ACCEPT).iter();
+        assert_eq!(all_values.next().unwrap(), &"application/json");
+        assert_eq!(all_values.next().unwrap(), &"application/xml");
+        assert_eq!(all_values.next(), None);
+    }
+
     #[test]
     fn add_body() {
         let client = Client::new();