The docs for ResponseWriter.Write say
// If the Header
// does not contain a Content-Type line, Write adds a Content-Type set
// to the result of passing the initial 512 bytes of written data to
// DetectContentType.

The header X-Content-Type-Options:nosniff is an explicit directive that
content-type should not be sniffed.

This changes the behavior of Response.WriteHeader so that, when
there is an X-Content-Type-Options:nosniff header, but there is
no Content-type header, the following happens:
1.  A Content-type:application/octet-stream is added
2.  A warning is logged via the server's logging mechanism.

Previously, a content-type would have been silently added based on
heuristic analysis of the first 512B which might allow a hosted
GIF like http://www.thinkfu.com/blog/gifjavascript-polyglots to be
categorized as JavaScript which might allow a CSP bypass, loading
as a script despite `Content-Security-Policy: script-src 'self' `.

----

https://fetch.spec.whatwg.org/#x-content-type-options-header
defines the X-Content-Type-Options header.

["Polyglots: Crossing Origins by Crossing Formats"](http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.905.2946&rep=rep1&type=pdf)
explains Polyglot attacks in more detail.

Change-Id: I2c8800d2e4b4d10d9e08a0e3e5b20334a75f03c0
Reviewed-on: https://go-review.googlesource.com/89275Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>