encoding/csv: truncate carriage returns at EOF

encoding/csv: truncate carriage returns at EOF

This fixes a regression where only CRLF was folded into LF at EOF.
Now, we also truncate trailing CR at EOF to preserve the old behavior.

Every one of the test cases added exactly matches the behavior
of Go1.9, even if the results are somewhat unexpected.

Fixes #22937

Change-Id: I1bc6550533163ae489ea77ec1e598163267b7eec
Reviewed-on: https://go-review.googlesource.com/81577
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>