Fix response headers getting overwritten in ASGI, WSGI protocols (#53)

src/asgi/io.rs

@@ -366,7 +366,7 @@ fn adapt_headers(message: &PyDict) -> HeaderMap {
                     HeaderName::from_bytes(tup[0]),
                     HeaderValue::from_bytes(tup[1])
                 ) {
-                    (Ok(key), Ok(val)) => { ret.insert(key, val); },
+                    (Ok(key), Ok(val)) => { ret.append(key, val); },
                     _ => {}
                 }
             };

src/wsgi/http.rs

@@ -37,7 +37,7 @@ macro_rules! handle_request {
                     let headers = res.headers_mut();
                     headers.insert(HK_SERVER, HV_SERVER);
                     for (key, val) in pyheaders {
-                        headers.insert(
+                        headers.append(
                             HeaderName::from_bytes(key.as_bytes()).unwrap(),
                             HeaderValue::from_str(&val).unwrap()
                         );