net/http: validate transmitted header fields

net/http: validate transmitted header fields

This makes sure the net/http package never attempts to transmit a
bogus header field key or value and instead fails fast with an error
to the user, rather than relying on the server to maybe return an
error.

It's still possible to use x/net/http2.Transport directly to send
bogus stuff. This change only stops h1 & h2 usage via the net/http
package. A future change will update x/net/http2.

This change also moves some code from request.go to lex.go, which in a
separate future change should be moved so it can be shared with http2
to reduce code bloat.

Updates #14048

Change-Id: I0a44ae1ab357fbfcbe037aa4b5d50669a87f2856
Reviewed-on: https://go-review.googlesource.com/21326
Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>