http: delete error kludge

http: delete error kludge

The kludge is targeted at broken web browsers
like Chrome and IE, but it gets in the way of
sending 400 or 500-series error results with
formatted bodies in response to AJAX requests
made by pages executing in those browsers.

Now the AJAX cases will work and Chrome
and IE will be as broken with Go servers as
they are with all the other servers.

Fixes #2169.

R=bradfitz, dsymonds
CC=golang-dev
https://golang.org/cl/4930047