net/http: allow server handlers to omit Content-Type

[bradfitz]

Complaint for proxy authors: there's no way for a Go HTTP Handler to return an HTTP
response without a Content-Type response header set.

We always auto-sniff one if missing.

And if blank, we send "Content-Type: \r\n" in the response, instead of
omitting the header. We should probably omit it in that case, like we do for the Date
header.

[gopherbot]

CL https://golang.org/cl/9412 mentions this issue.