**Describe the bug**
When using https://github.com/axios/axios/pull/1712 example and adding baseURL in config, baseURL seems to not be used.

**To Reproduce**

```js
const axios = require("axios");
const fakeConfig = {
  method: "post",
  url: "/user/12345",
  baseURL: "/foo"
};
console.log(axios.getUri(fakeConfig));
```

**Expected behavior**
`axios.getUri` should return `/foo/user/12345`.

If this expected behavior seems right, I can work on a fix.

**Environment:**
 - Axios Version: 0.19.0
 - OS: Linux, Windows
 - Browser Chrome but also happen on Firefox and node
test/specs/instance.spec.js
it('should make an http request with url instead of baseURL', function (done) {
    var instance = axios.create({
      url: 'https://api.example.com'
    });
    instance('/foo');
    getAjaxRequest().then(function (request) {
      expect(request.url).toBe('/foo');
      done();
    });
  });
  it('should make an http request', function (done) {
    var instance = axios.create();
    instance.get('/foo');
    getAjaxRequest().then(function (request) {
      expect(request.url).toBe('/foo');
      done();
    });
  });
  it('should use instance options', function (done) {
    var instance = axios.create({ timeout: 1000 });
    instance.get('/foo');
    getAjaxRequest().then(function (request) {
      expect(request.timeout).toBe(1000);
      done();
    });
  });
  it('should have defaults.headers', function () {
    var instance = axios.create({
      baseURL: 'https://api.example.com'
    });