style(lib): add must_use attributes to futures and streams

src/client/connect.rs

@@ -147,6 +147,7 @@ impl StdError for InvalidUrl {
 }
 
 /// A Future representing work to connect to a URL.
+#[must_use = "futures do nothing unless polled"]
 pub struct HttpConnecting {
     state: State,
     handle: Handle,

src/client/mod.rs

@@ -110,6 +110,7 @@ where C: Connect,
 }
 
 /// A `Future` that will resolve to an HTTP Response.
+#[must_use = "futures do nothing unless polled"]
 pub struct FutureResponse(Box<Future<Item=Response, Error=::Error> + 'static>);
 
 impl fmt::Debug for FutureResponse {