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**Describe the bug**

Param with value of null is replaced with "{}"

**To Reproduce**

axios.get('/whatever', {params: {test: null}}) used to make GET request to /whatever, now it's /whatever?test=%7B%7D

**Expected behavior**

Should request to /whatever.

**Environment:**
 - Axios Version [the current master]

**Additional context/Screenshots**

See https://github.com/axios/axios/pull/2656#issuecomment-598269460.
lib/core/Axios.js
*
 * @param {Object} config The config specific for this request (merged with this.defaults)
 */
Axios.prototype.request = function request(config) {
  
  
  if (typeof config === 'string') {
    config = arguments[1] || {};
    config.url = arguments[0];
  } else {
    config = config || {};
  }
  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 chain = [dispatchRequest, undefined];
  var promise = Promise.resolve(config);
  this.interceptors.request.forEach(function unshiftRequestInterceptors(interceptor) {
    chain.unshift(interceptor.fulfilled, interceptor.rejected);
  });
  this.interceptors.response.forEach(function pushResponseInterceptors(interceptor) {
    chain.push(interceptor.fulfilled, interceptor.rejected);
  });