If a handler replies to a HEAD request with a Content-Length field, the *Response
contains the "Content-Length: 1234" header, but the ContentLength field is
zero.

The docs say:

type Response struct {
...
    // ContentLength records the length of the associated content.  The
    // value -1 indicates that the length is unknown.  Unless RequestMethod
    // is "HEAD", values >= 0 indicate that the given number of bytes may
    // be read from Body.
    ContentLength int64
}

... suggesting that we should be setting it when we know it.

Related, Response.Write should write the correct length.  Currently the one in the
header map is being ignored in favor of the zero.