req.wantsJson logic is not robust, leads to errors in SEO

**Node version**: 14.20.0
**Sails version** _(sails)_: 1.5.2

`req.wantsJson` logic is not robust as it returns true when `accept` header is `*/*` and has no `content-type` or `x-requested-with`.
This leads to error in SEO when I use this property to return json vs html. In the case (`accept: */*`), what search engine crawlers use, they expect html. But as wantsJson is true, my code returns json.

We could overcome this if we default wantsJson to false when it is not clear whether requested resource is json or not.

Start by tracing how Sails determines req.wantsJson from the Accept, Content-Type, and X-Requested-With headers. Reproduce the reported case with Accept: */* and no other indicators, then inspect existing tests around this request behavior. Done means the agreed handling of ambiguous requests is implemented and covered by tests.

Written by the indexing model from the issue text.