Fix bypass proxy middleware

[SpaceK33z]

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 #359, and effects 1.15.0 and the 2.x branch.

For now, this is based on 1.15.0, because the people that reported issues with this are using that version. It should be merged to master too.

[SpaceK33z]

Verified to work in #560 by @cletusw.

[HZZformGD]

@SpaceK33z excuse me , can i ask u a question about webpack-dev-server, my English is not good, so hope you can know what i say, my problem is when i npm run dev , i got this error

D:\nodejs\vue+koa+MD\myblog\node_modules\http-proxy-middleware\lib\config-factory.js:42

        throw new Error('[HPM] Missing "target" option. Example: {target: "http://www.e
xample.org"}');

i google this, and found someone has this trouble , his trouble is similar to mine.
And then i follow the suggestion——copy the lib/server.js which is fixed ,and run again , unluckyly, i got the same error again , ps: i can run node app.js ,it works on chrome, it also works when i npm run build,