This improves our cache consistency when the charset declaration is not within the first 1024 bytes of the document, properly re-fetching resources from the net where we should.
If content-type is not sent by the server, then there's nothing to check against, and we should allow the load.
"nosniff" headers are normally sent as a blanket security measure on a domain, but not every server response will include an explicit MIME type.
Follow-up for #1343.
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