[5.2] Confirm Ajax request is not Pjax

[dwightwatson]

This brings the response requirements in the FormRequest into line with the ValidatesRequest trait. See issue #13997 for more information.

[barryvdh]

And just checking the wantsJson, or is that not reliable enough?

[sebastiandedeyne]

I'm actually a fan of only checking wantsJson everywhere, would make it cleaner to request html through javascript when you're not in a pjax context. Not sure if that would break existing implementations though.

[dwightwatson]

I was just bringing the constraints in the FormRequest into line with the ValidatesRequest trait. I suppose there is an argument to only check wantsJson, though it would not be backwards compatible.

[taylorotwell]

Merging this for now but do have in my notes to revisit all these types of checks where we seem to just need one method.