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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*

lib/core/dispatchRequest.js
function cleanHeaderConfig(method) {
      delete config.headers[method];
    }
  );
  var adapter = config.adapter || defaults.adapter;
  return adapter(config).then(function onAdapterResolution(response) {
    throwIfCancellationRequested(config);
    
    response.data = transformData(
      response.data,
      response.headers,
      config.transformResponse
    );
    return response;
  }, function onAdapterRejection(reason) {
    if (!isCancel(reason)) {
      throwIfCancellationRequested(config);
      
      if (reason && reason.response) {
        reason.response.data = transformData(
          reason.response.data,
          reason.response.headers,
          config.transformResponse
        );
      }
    }
    return Promise.reject(reason);
  });
};