devServer.proxy config does not accept object as described in options schema

[petterm]

• Operating System: macOS High Sierra 10.13.5
• Node Version: 10.5.0
• NPM Version: 6.1.0
• webpack Version: 4.13.0
• webpack-dev-server Version: 3.1.4

• This is a bug
• This is a modification request

Code

const webpack = require('webpack');
const path = require('path');

const configuration = {
    entry: path.resolve(__dirname, 'src/source.js'),
    output: {
        filename: 'bundle.js',
        path: path.resolve(__dirname, 'dist'),
    },
    mode: 'development',
    devServer: {
        contentBase: false,
        host: 'localhost',
        proxy: {
            context: () => true,
            target: 'http://localhost:8080',
        },
        port: 8000,
    },
};

module.exports = configuration;

Expected Behavior

Dev server should proxy request http://localhost:8000/test to http://localhost:8080/test.

Actual Behavior

Dev server does not recognise the proxy setup and returns Cannot GET /test

For Bugs; How can we reproduce the behavior?

If the proxy config is changed to an array with the current object as a single value it works as intended. The initial config should be valid though following the options schema.

Minimal repository to reproduce error: https://github.com/petterm/webpack-dev-server-test