Testing and cleanup of transformResponse (#3377)

- Tests for transformResponse
 - Remove eslint error by renaming the var
 - Test that there data a length to avoid JSON.parse headache
 - Use `util.isString()` over `typeof`

Co-authored-by: Jay <jasonsaayman@gmail.com>

lib/defaults.js

@@ -55,13 +55,13 @@ var defaults = {
   }],
 
   transformResponse: [function transformResponse(data) {
-    /*eslint no-param-reassign:0*/
-    if (typeof data === 'string') {
+    var result = data;
+    if (utils.isString(result) && result.length) {
       try {
-        data = JSON.parse(data);
+        result = JSON.parse(result);
       } catch (e) { /* Ignore */ }
     }
-    return data;
+    return result;
   }],
 
   /**