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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
});
  afterEach(function () {
    jasmine.Ajax.uninstall();
    axios.interceptors.request.handlers = [];
    axios.interceptors.response.handlers = [];
  });
  it('should add a request interceptor', function (done) {
    axios.interceptors.request.use(function (config) {
      config.headers.test = 'added by interceptor';
      return config;
    });
    axios('/foo');
    getAjaxRequest().then(function (request) {
      request.respondWith({
        status: 200,
        responseText: 'OK'
      });
      expect(request.requestHeaders.test).toBe('added by interceptor');
      done();
    });
  });
  it('should add a request interceptor that returns a new config object', function (done) {
    axios.interceptors.request.use(function () {
      return {
        url: '/bar',
        method: 'post'
      };
    });
    axios('/foo');
    getAjaxRequest().then(function (request) {