Fix bypass proxy middleware (webpack#563)

The `bypass` feature was no longer working because the code added the bypass method as middleware AND `httpProxyMiddleware` as middleware. However, if a `bypass` method is given, it should only use that as middleware.

This was caused in webpack#359, and effects `1.15.0` and the 2.x branch.

lib/Server.js

@@ -170,19 +170,25 @@ function Server(compiler, options) {
 				 * ]
 				 */
 				options.proxy.forEach(function(proxyConfig) {
+					var bypass = typeof proxyConfig.bypass === 'function';
 					var context = proxyConfig.context || proxyConfig.path;
+					var proxyMiddleware;
+					// It is possible to use the `bypass` method without a `target`.
+					// However, the proxy middleware has no use in this case, and will fail to instantiate.
+					if(proxyConfig.target) {
+						proxyMiddleware = httpProxyMiddleware(context, proxyConfig);
+					}
 
 					app.use(function(req, res, next) {
-						if(typeof proxyConfig.bypass === 'function') {
-							var bypassUrl = proxyConfig.bypass(req, res, proxyConfig) || false;
+						var bypassUrl = bypass && proxyConfig.bypass(req, res, proxyConfig) || false;
 
-							if(bypassUrl) {
-								req.url = bypassUrl;
-							}
+						if(bypassUrl) {
+							req.url = bypassUrl;
+							next();
+						} else if(proxyMiddleware) {
+							return proxyMiddleware(req, res, next);
 						}
-
-						next();
-					}, httpProxyMiddleware(context, proxyConfig));
+					});
 				});
 			}
 		},