fix historyApiFallback+proxy combo

[Lendar]

Both contentBase: "http://url" and proxy are incompatible with historyApiFallback. This PR fixes that.

Steps to reproduce:

Setup webpack with config

var server = new WebpackDevServer(webpack(config), {
  historyApiFallback: true,
  proxy: {
    "/api/*": "http://localhost:3100/"
  }
})

Then, open /api/foo in browser.

Expected result: result from proxy is returned.
Actual result: index.html is returned.

[sokra]

Thanks