net/http: Various fixes to Basic authentication

There were some issues with the code sometimes using base64.StdEncoding,
and sometimes base64.URLEncoding.
Encoding basic authentication is now always done by the same code.

Fixes issue 5970.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/12397043

Committer: Brad Fitzpatrick <bradfitz@golang.org>