Fix exception when using multicompiler.

Fixes #125

middleware.js

@@ -56,7 +56,7 @@ module.exports = function(compiler, options) {
 
 	// store our files in memory
 	var fs;
-	var isMemoryFs = compiler.outputFileSystem instanceof MemoryFileSystem;
+	var isMemoryFs = !compiler.compilers && compiler.outputFileSystem instanceof MemoryFileSystem;
 	if(isMemoryFs) {
 		fs = compiler.outputFileSystem;
 	} else {

test/Server.test.js

@@ -4,6 +4,7 @@ var webpack = require("webpack");
 var should = require("should");
 var request = require("supertest");
 var webpackConfig = require("./fixtures/server-test/webpack.config");
+var webpackMultiConfig = require("./fixtures/server-test/webpack.array.config");
 
 
 describe("Server", function() {
@@ -145,6 +146,30 @@ describe("Server", function() {
 		});
 	});
 
+	describe("MultiCompiler", function() {
+		before(function(done) {
+			app = express();
+			var compiler = webpack(webpackMultiConfig);
+			var instance = middleware(compiler, {
+				stats: "errors-only",
+				quiet: true,
+				publicPath: "/",
+			});
+			app.use(instance);
+			listen = listenShorthand(done);
+		});
+		after(close);
+
+		it("request to both bundle files", function(done) {
+			request(app).get("/foo.js")
+			.expect(200, function() {
+				request(app).get("/bar.js")
+				.expect(200, done);
+			});
+		});
+	});
+
+
 	describe("server side render", function() {
 		var locals;
 		before(function(done) {

test/fixtures/server-test/bar.js

@@ -0,0 +1 @@
+console.log("Bar");

test/fixtures/server-test/webpack.array.config.js

@@ -0,0 +1,15 @@
+module.exports = [{
+	context: __dirname,
+	entry: "./foo.js",
+	output: {
+		filename: "foo.js",
+		path: "/"
+	}
+}, {
+	context: __dirname,
+	entry: "./bar.js",
+	output: {
+		filename: "bar.js",
+		path: "/"
+	}
+}];

test/fixtures/server-test/webpack.config.js

@@ -1,6 +1,6 @@
 module.exports = {
 	context: __dirname,
-	entry: "./index.js",
+	entry: "./foo.js",
 	output: {
 		filename: "bundle.js",
 		path: "/"