encoding/csv: always allow trailing commas

Original CL by rsc (11916045):

The motivation for disallowing them was RFC 4180 saying
"The last field in the record must not be followed by a comma."
I believe this is an admonition to CSV generators, not readers.
When reading, anything followed by a comma is not the last field.

Fixes issue 5892.

R=golang-dev, rsc, r
CC=golang-dev
https://codereview.appspot.com/12294043

Committer: Rob Pike <r@golang.org>