**To Reproduce**
Code snippet to reproduce, ideally that will work by pasting into something like https://npm.runkit.com/axios, a hosted solution, or a repository that illustrates the issue. 

```js
const axios = require("axios")

const url = 'https://stackoverflow.com/questions';

const {headers} = await axios.head(url, {
  headers: {'accept-encoding': 'gzip, deflate, br'}
});
```
You can run this in one click here https://runkit.com/embed/eocb5097ds6r
 
**Expected behavior**
Don't crush

**Environment:**
 - Node.js any version

**Solution:**
It require trivial fix similar to https://github.com/axios/axios/pull/1129
lib/adapters/http.js
'use strict';
var utils = require('./../utils');
var settle = require('./../core/settle');
var buildFullPath = require('../core/buildFullPath');
var buildURL = require('./../helpers/buildURL');
var http = require('http');
var https = require('https');
var httpFollow = require('follow-redirects').http;
var httpsFollow = require('follow-redirects').https;
var url = require('url');
var zlib = require('zlib');
var pkg = require('./../../package.json');
var createError = require('../core/createError');
var enhanceError = require('../core/enhanceError');
var isHttps = /https:?/;
module.exports = function httpAdapter(config) {
  return new Promise(function dispatchHttpRequest(resolvePromise, rejectPromise) {
    var resolve = function resolve(value) {
      resolvePromise(value);
    };
    var reject = function reject(value) {
      rejectPromise(value);
    };
    var data = config.data;
    var headers = config.headers;
    
    
    
    if (!headers['User-Agent'] && !headers['user-agent']) {
      headers['User-Agent'] = 'axios/' + pkg.version;