net/http: enable Transfer-Encoding: identity without Content-Length for HTTP 1.1.

Use case is SSE recommended configuration: http://www.w3.org/TR/eventsource/#notes
Removes a TODO.

LGTM=bradfitz
R=golang-codereviews, bradfitz, tommi.virtanen
CC=golang-codereviews
https://codereview.appspot.com/100000044

Committer: Brad Fitzpatrick <bradfitz@golang.org>