Add test for WindowProxy to implement Send

tests/window_proxy_send.rs

@@ -0,0 +1,9 @@
+extern crate glutin;
+
+#[cfg(feature = "window")]
+#[test]
+fn window_proxy_send() {
+    // ensures that `glutin::WindowProxy` implements `Send`
+    fn needs_send<T:Send>() {}
+    needs_send::<glutin::WindowProxy>();
+}