#### Is your feature request related to a problem? Please describe.
Some APIs only work if the User-Agent header is entirely omitted.  This is not currently possible in Axios in NodeJS.  #1411 opened this discussion some time back, but was closed without an actual solution other than commenting out the code which inserts the header.

#### Describe the solution you'd like
There should be a way to completely omit the U-A header if desired.  User-Agent is not a required header, and its deprecation (in favor of Client-Hints) is being actively discussed.

The simplest solution would probably to retain the current behavior if no User-Agent header is specified, but omit the header if it is explicitly provided with a falsy value (`null` and `""` both seem to clearly communicate the caller's intent not to have a value here).

#### Describe alternatives you've considered
Explicit options to disable the current behavior could work, but that seems like a less-elegant path.

#### Additional context
No.

test/unit/adapters/http.js
res.setHeader('Content-Encoding', 'gzip');
        res.end(zipped);
      }).listen(4444, function() {
        axios.get('http://localhost:4444/', {
          decompress: false,
          responseType: 'arraybuffer'
        }).then(function(res) {
          assert.equal(res.data.toString('base64'), zipped.toString('base64'));
          done();
        });
      });
    });
  });
  it('should support UTF8', function (done) {
    var str = Array(100000).join('ж');
    server = http.createServer(function (req, res) {
      res.setHeader('Content-Type', 'text/html; charset=UTF-8');
      res.end(str);
    }).listen(4444, function () {
      axios.get('http://localhost:4444/').then(function (res) {
        assert.equal(res.data, str);
        done();
      });
    });
  });
  it('should support basic auth', function (done) {
    server = http.createServer(function (req, res) {
      res.end(req.headers.authorization);
    }).listen(4444, function () {
      var user = 'foo';