<!-- Click "Preview" for a more readable version -->


#### Summary

set baseURL in interceptors is not working.

```
const service = axios.create({
  baseURL: 'http://localhost/'
});

service.interceptors.request.use(config => {

  config.baseURL = 'dd'
  console.log(config.url) // output :  http://localhost/././../

  return config;
}, error => {
  // Do something with request error
  console.error(error); // for debug
  Promise.reject(error);
})
```

#### Context

- axios version: *e.g.: v0.16.2*
- Environment: *e.g.: node vv8.0.0, chrome  59.0.3071.86, macOS 10.12*

test/specs/interceptors.spec.js
expect(request.method).toBe('POST');
      expect(request.url).toBe('/bar');
      done();
    });
  });
  it('should add a request interceptor that returns a promise', function (done) {
    axios.interceptors.request.use(function (config) {
      return new Promise(function (resolve) {
        
        setTimeout(function () {
          config.headers.async = 'promise';
          resolve(config);
        }, 100);
      });
    });
    axios('/foo');
    getAjaxRequest().then(function (request) {
      expect(request.requestHeaders.async).toBe('promise');
      done();
    });
  });
  it('should add multiple request interceptors', function (done) {
    axios.interceptors.request.use(function (config) {
      config.headers.test1 = '1';
      return config;
    });
    axios.interceptors.request.use(function (config) {
      config.headers.test2 = '2';
      return config;
    });