Update grouped-categories.js deepClone function

(Sorry if this not correct process for contributing on GitHub, feel free to let me know the best way to do this in the future.)
By the way great plugin :+1: 

Updated deepClone function prevent infinite loop when final "category" property is an object.

Previously this function would only complete successfully if the final "category" property was an array of strings.
ie.
xAxis: {
   categories: [{
       name: "1 & 2 Broadgate",
       categories: ['Sub Category 1', 'Sub Category 2']
   }...

This change allow it also to accept objects within final category layer:
xAxis: {
   categories: [{
       name: "Category 1",
       categories: [{ name: "Sub Category 1" }, { name: "Sub Category 2" }]
   }...