fix(proxy): add connection keep-live option proxy

[fouad-j]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Underlying tools
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

Other information:

I faced a proxy issue for POST call with body contains more than 40.000 characters.

this the error I got:

[1] Proxy error: Could not proxy request /rest/v1/hello from localhost:8080 to http://localhost:4300/.
[1] See https://nodejs.org/api/errors.html#errors_common_system_errors for more information (ECONNRESET).
[1] [HPM] Error occurred while trying to proxy request /rest/v1/hello from localhost:8080 to http://localhost:4300/ (ECONNRESET) (https://nodejs.org/api/errors.html#errors_common_system_errors)

Steps to reproduce

vue.config.js

module.exports = {
  publicPath: '.',
  devServer: {
    proxy: {
      '(/rest/.*|/v1/.*)': {
        target: 'http://localhost:4300',
        secure: false,
        logLevel: 'debug'
      }
    }
  }
};

server express

const express = require('express');
const router = express.Router();
const app = express();

router
  .get('/rest/v1/hello', (req, res) => res.send({}));

app
  .use(router)
  .listen(4300, () => console.log('App listening at http://localhost:%s', 4300));

call from my vue.js application

const body = {};/*more than 40.000 characters*/
axios
  .post('/rest/v1/hello', body)
  .then(response => response.data)
  .then(test => commit({type: 'test', test}));

after debug, I remarked that the proxy request was "Connection: close"

So I change it to 'Connection: keep-alive' it fixed this issue

I think this option should be add to proxy default value