Make bundler middleware usage intuitive (#449)

* Make bundler middleware usage intuitive

* Fix middleware test

Specifically bundling `/commonjs/index.js` in the middleware test before running
the `should produce two bundles when importing a CSS file` one
seems to cause `bunde()` in the latter to throw.

src/Bundler.js

@@ -473,6 +473,7 @@ class Bundler extends EventEmitter {
   }
 
   middleware() {
+    this.bundle();
     return Server.middleware(this);
   }
 

test/bundler.js

@@ -0,0 +1,14 @@
+const assert = require('assert');
+const {bundler} = require('./utils');
+
+describe('bundler', function() {
+  it('should bundle once before exporting middleware', function(done) {
+    let b = bundler(__dirname + '/integration/bundler-middleware/index.js');
+    b.middleware();
+
+    setTimeout(() => {
+      assert(b.mainAsset);
+      done();
+    }, 300);
+  });
+});

test/integration/bundler-middleware/index.js

@@ -0,0 +1,5 @@
+var local = require('./local');
+
+module.exports = function () {
+  return local.a + local.b;
+};

test/integration/bundler-middleware/local.js

@@ -0,0 +1,2 @@
+exports.a = 1;
+exports.b = 2;