Switch property to cached_property

[vsalvino]

Fixes #69

Should lead to a slight performance improvement.

[Prill]

@vsalvino Came here from #69. Poking around at the failing test_struct_org test, one thing I noticed is that you are applying @cached_property to methods that return mutable objects (seo_struct_org_base_dict and seo_struct_org_dict). But seo_struct_org_dict calls update on the value that is returned from seo_struct_org_base_dict, meaning that it's updating the cached value that the base property returns. I'm not sure if that's what's causing the specific error since the output doesn't show the actual response contents, but that seems incorrect regardless.

I'm not sure what the best solution overall is, but theoretically you could apply the @cached_property decorate only on properties that return immutable values, like seo_struct_org_base_json and seo_struct_org_json.