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#### 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
axios.interceptors.request.use(function (config) {
      config.headers.test3 = '3';
      return config;
    });
    axios('/foo');
    getAjaxRequest().then(function (request) {
      expect(request.requestHeaders.test1).toBe('1');
      expect(request.requestHeaders.test2).toBe('2');
      expect(request.requestHeaders.test3).toBe('3');
      done();
    });
  });
  it('should add a response interceptor', function (done) {
    var response;
    axios.interceptors.response.use(function (data) {
      data.data = data.data + ' - modified by interceptor';
      return data;
    });
    axios('/foo').then(function (data) {
      response = data;
    });
    getAjaxRequest().then(function (request) {
      request.respondWith({
        status: 200,
        responseText: 'OK'
      });
      setTimeout(function () {
        expect(response.data).toBe('OK - modified by interceptor');
        done();
      }, 100);