Alpha2 ISO country code for Namibia (NA) is silently discarded

Created by: eric-brechemier

This is quite unexpected. Probably related to #49 (closed).

Minimal example:

alpha2.csv

name,ISO3166-1-Alpha-2
Namibia,NA

csvstat: Most common values: None

$ csvstat alpha2.csv 
  1. "name"

	Type of data:          Text
	Contains null values:  False
	Unique values:         1
	Most common values:    Namibia (1x)

  2. "ISO3166-1-Alpha-2"

	Type of data:          Boolean
	Contains null values:  True (excluded from calculations)
	Unique values:         1
	Most common values:    None (1x)

Row count: 1

csvjoin: "NA" is silently replaced with ""

$ csvjoin -c1,1 alpha2.csv alpha2.csv 
name,ISO3166-1-Alpha-2,ISO3166-1-Alpha-22
Namibia,,