#### 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('Location', '/foo');
      res.statusCode = 302;
      res.end();
    }).listen(4444, function () {
      axios.get('http://localhost:4444/', {
        maxRedirects: 0,
        validateStatus: function () {
          return true;
        }
      }).then(function (res) {
        assert.equal(res.status, 302);
        assert.equal(res.headers['location'], '/foo');
        done();
      });
    });
  });
  it('should support max redirects', function (done) {
    var i = 1;
    server = http.createServer(function (req, res) {
      res.setHeader('Location', '/' + i);
      res.statusCode = 302;
      res.end();
      i++;
    }).listen(4444, function () {
      axios.get('http://localhost:4444/', {
        maxRedirects: 3
      }).catch(function (error) {
        done();
      });
    });