This moves the checking code back to NS_SniffContent and specifically
checks requests for either:
- Empty `Content-type` header, OR
- A known JSON MIME type
If present, sniffing is allowed despite `nosniff` because in the first
case this is webmaster error (conflicting `nosniff` without MIME type
would always fail, so sniff it), and in the second case we need this
carveout or our json viewer will break.
This fixes a regression for the JSON viewer from part 1 as it relies on
sniffing to prettify (and should carve out the exception even if `nosniff`
headers are sent).
No real functional changes otherwise. Just catering to a corner case.
