All the readers are denoted as `b` while for `Reader.Size()` it is `r`.

Change-Id: Ib6f97306c11b3abb2ff30edbc9f9362cad36d080
GitHub-Last-Rev: 992f88b374b5a309303b7fa1622ee629d0fb741b
GitHub-Pull-Request: golang/go#26205
Reviewed-on: https://go-review.googlesource.com/122156Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>