doc/codewalk: gofmt pig.go

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7221079
net/url: normalize scheme to lower case (http not HTTP)

Also document %2f vs / ambiguity in URL.Path.

Fixes issue 3913.
Fixes issue 3659.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7225076
io: guarantee err == nil for full reads in ReadFull and ReadAtLeast

This is a backwards compatible API change that fixes broken code.

In Go 1.0, ReadFull(r, buf) could return either len(buf), nil or len(buf), non-nil.
Most code expects only the former, so do that and document the guarantee.

Code that was correct before is still correct.
Code that was incorrect before, by assuming the guarantee, is now correct too.

The same applies to ReadAtLeast.

Fixes issue 4544.

R=golang-dev, bradfitz, minux.ma
CC=golang-dev
https://codereview.appspot.com/7235074