Rename 'unstable-stream' feature to 'stream' (#733)

tests/multipart.rs

@@ -55,7 +55,7 @@ async fn text_part() {
     assert_eq!(res.status(), reqwest::StatusCode::OK);
 }
 
-#[cfg(feature = "unstable-stream")]
+#[cfg(feature = "stream")]
 #[tokio::test]
 async fn stream_part() {
     use futures_util::{future, stream};