fix set leverage for resetting back to cross (value == 0)

lib/rest-client.js

@@ -184,9 +184,12 @@ module.exports = class RestClient {
 
   async changeUserLeverage(params) {
     assert(params, 'No params passed');
-    assert(params.leverage, 'Parameter leverage is required');
     assert(params.symbol, 'Parameter symbol is required');
 
+    if (typeof params.leverage === 'undefined') {
+      throw new Error('Parameter leverage is required');
+    }
+
     return await this.request.post('user/leverage/save', params);
   }