**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
expect(typeof instance.defaults.headers, 'object');
    expect(typeof instance.defaults.headers.common, 'object');
  });
  it('should have interceptors on the instance', function (done) {
    axios.interceptors.request.use(function (config) {
      config.foo = true;
      return config;
    });
    var instance = axios.create();
    instance.interceptors.request.use(function (config) {
      config.bar = true;
      return config;
    });
    var response;
    instance.get('/foo').then(function (res) {
      response = res;
    });
    getAjaxRequest().then(function (request) {
      request.respondWith({
        status: 200
      });
      setTimeout(function () {
        expect(response.config.foo).toEqual(undefined);
        expect(response.config.bar).toEqual(true);
        done();
      }, 100);
    });
  });
});