**Describe the bug**
When using https://github.com/axios/axios/pull/1712 example and adding baseURL in config, baseURL seems to not be used.

**To Reproduce**

```js
const axios = require("axios");
const fakeConfig = {
  method: "post",
  url: "/user/12345",
  baseURL: "/foo"
};
console.log(axios.getUri(fakeConfig));
```

**Expected behavior**
`axios.getUri` should return `/foo/user/12345`.

If this expected behavior seems right, I can work on a fix.

**Environment:**
 - Axios Version: 0.19.0
 - OS: Linux, Windows
 - Browser Chrome but also happen on Firefox and node
lib/core/Axios.js
}
  config = mergeConfig(this.defaults, config);
  
  if (config.method) {
    config.method = config.method.toLowerCase();
  } else if (this.defaults.method) {
    config.method = this.defaults.method.toLowerCase();
  } else {
    config.method = 'get';
  }
  var transitional = config.transitional;
  if (transitional !== undefined) {
    validator.assertOptions(transitional, {
      silentJSONParsing: validators.transitional(validators.boolean),
      forcedJSONParsing: validators.transitional(validators.boolean),
      clarifyTimeoutError: validators.transitional(validators.boolean)
    }, false);
  }
  
  var requestInterceptorChain = [];
  var synchronousRequestInterceptors = true;
  this.interceptors.request.forEach(function unshiftRequestInterceptors(interceptor) {
    if (typeof interceptor.runWhen === 'function' && interceptor.runWhen(config) === false) {
      return;
    }
    synchronousRequestInterceptors = synchronousRequestInterceptors && interceptor.synchronous;
    requestInterceptorChain.unshift(interceptor.fulfilled, interceptor.rejected);
  });
  var responseInterceptorChain = [];
  this.interceptors.response.forEach(function pushResponseInterceptors(interceptor) {