#### 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
}).listen(4444, function () {
      axios.post('http://localhost:4444/',
        fs.createReadStream(notExitPath)
      ).then(function (res) {
        assert.fail();
      }).catch(function (err) {
        assert.equal(err.message, `ENOENT: no such file or directory, open \'${notExitPath}\'`);
        done();
      });
    });
  });
  it('should support buffers', function (done) {
    var buf = Buffer.alloc(1024, 'x'); // U
    server = http.createServer(function (req, res) {
      assert.equal(req.headers['content-length'], buf.length.toString());
      req.pipe(res);
    }).listen(4444, function () {
      axios.post('http://localhost:4444/',
        buf, {
          responseType: 'stream'
        }).then(function (res) {
          var stream = res.data;
          var string = '';
          stream.on('data', function (chunk) {
            string += chunk.toString('utf8');
          });
          stream.on('end', function () {
            assert.equal(string, buf.toString());
            done();
          });