**Describe the bug**

Since I upgrade axios from 0.18.0 to 0.19.0, the requests made to fetch a local file (`file:///`) with a relative path return an error: ```Request failed with status code 0```.  However the web browser will always return a status code 0 when the request success because there is no HTTP server involved. The XHR readyState is also 4, if there was a network error, I believe this one would probably be 0 as well.

**To Reproduce**
```js
// In a HTML file like opened at file:///tmp/axios.html for example
axios.get('./file.ttf', {
  responseType: 'arraybuffer',
}).then((resp) => {
  console.log('OK', resp);
}).catch((err) => {
  if (err.request && err.request.readyState === 4) {
     //  The response here should not be treated as an error
  }
  console.log('ERR', err);
});
```

**Expected behavior**
The request should success and return the arraybuffer (works with axios 0.18.0).

**Environment:**
 - Axios Version: 0.19.0
 - OS: iOS 12.4, OSX 10.14.6
 - Browser: Safari
 - Browser Version: 12.1.2
 - Additional Library Versions: None

**Additional context/Screenshots**
![Screenshot 2019-09-07 at 13 26 37](https://user-images.githubusercontent.com/396537/64474302-b8992800-d173-11e9-999e-5cf19eaf678b.png)
![Screenshot 2019-09-07 at 13 31 10](https://user-images.githubusercontent.com/396537/64474306-ccdd2500-d173-11e9-93da-990fef265a18.png)

test/specs/requests.spec.js
}).catch(function (error) {
        config = error.config;
    });
    getAjaxRequest().then(function (request) {
      request.respondWith({
        status: 404,
        statusText: 'NOT FOUND',
        responseText: 'Resource not found'
      });
      setTimeout(function () {
        expect(config.baseURL).toEqual('/api');
        expect(config.url).toEqual('/foo');
        done();
      }, 100);
    });
  });
  it('should allow overriding Content-Type header case-insensitive', function (done) {
    var response;
    var contentType = 'application/vnd.myapp.type+json';
    axios.post('/foo', { prop: 'value' }, {
      headers: {
        'content-type': contentType
      }
    }).then(function (res) {
      response = res;
    });
    getAjaxRequest().then(function (request) {
      expect(request.requestHeaders['Content-Type']).toEqual(contentType);
      done();
    });