Right now, a ResponseWriter Write will return ErrBodyNotAllowed for a HEAD request.

While some applications may want to check Request.Method == "HEAD" to avoid
some computation, the majority would prefer to be lazy.

For instance, see issue #5451 and the proposed fix
(https://golang.org/cl/9388043/) to shut up some bogus warnings:

        if err := godocHTML.Execute(w, page); err != nil && err != http.ErrBodyNotAllowed {
                log.Printf("godocHTML.Execute: %s", err)
    }

That means applications have to care about HEAD vs GET.

The server's default behavior should shield users from caring about HEAD and just do the
right thing, which means:

-- don't return ErrBodyNotAllowed for HEAD requests. (we'll still return it for other
cases)
-- still do Content-Type sniffing on first 512 bytes.
-- still count Content-Length and send that header, for bodies <= 2KB

Currently we don't do any of those 3, which means our response headers to HEAD requests
aren't the same as our responses to GET requests, unless the app goes out of its way to
do so, which most don't, since HEAD requests are rare.