code: function(options) {
          _.each(options, function(value, key) {
            if (value === undefined) {
              delete options[key];
            } else if (_.isObject(value)) {
              this.cleanRequestOptions(value);
            }
          }, this);
        }
docs: With both `extendDefaultRequestOptions` and `mergeDefaultRequestOptions`, setting an option to `undefined` removes it.  this.setDefaultRequestOptions({ json: true, headers: { foo: 'bar', bar: 'baz' } });  this.extendDefaultRequestOptions({ json: undefined });  this.mergeDefaultRequestOptions({ headers: { foo: undefined } });  // default request options are now { //   headers: { bar: 'baz' } // }