The contract for Read specifies that a caller should process n bytes of
data before handling an error value. The previous implementation may
have discarded the final data read from the underlying reader if err
was non nil.

Change-Id: I81547cd9ea62c4060f79523679beed6bbc6ccd95
Reviewed-on: https://go-review.googlesource.com/23534Reviewed-by: Ian Lance Taylor <iant@golang.org>