Support Svelte 3

fixes #546, #551, #552, #554

test/apps/session/src/server.js

@@ -0,0 +1,19 @@
+import polka from 'polka';
+import * as sapper from '@sapper/server';
+
+const { PORT } = process.env;
+
+polka()
+	.use((req, res, next) => {
+		req.hello = 'hello';
+		res.locals = { name: 'world' };
+		next();
+	})
+	.use(
+		sapper.middleware({
+			session: (req, res) => ({
+				title: `${req.hello} ${res.locals.name}`
+			})
+		})
+	)
+	.listen(PORT);