Prevent failed circular reference detection in _.defaultsDeep. [closes #2537]

lodash.js

@@ -6115,7 +6115,6 @@
         // Recursively merge objects and arrays (susceptible to call stack limits).
         stack.set(srcValue, objValue);
         baseMerge(objValue, srcValue, undefined, mergeDefaults, stack);
-        stack['delete'](srcValue);
       }
       return objValue;
     }