The record delimiter (not configurable by user) is "\r\n" or "\n".
It is insensible for the user to set Comma or Comment delimiters
to be some character that conflicts with the record delimiter.
Furthermore, it is insensible for Comma or Comment to be the same rune.
Allowing this leaks implementation details to the user in regards to
the evaluation order of which rune is checked for first.

Fixes #22404

Change-Id: I31e86abc9b3a8fb4584e090477795587740970ae
Reviewed-on: https://go-review.googlesource.com/72793Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>