docs: public com.squareup.okhttp.Call getHeadersAsync(final ApiCallback<Map<String, String>> callback)
              throws ApiException {
  
          com.squareup.okhttp.Call call = getHeadersValidateBeforeCall(callback);
          Type localVarReturnType = new TypeToken<Map<String, String>>() {
          }.getType();
          apiClient.executeAsync(call, localVarReturnType, callback);
          return call;
      }