Revisit fix for #169

v0.12.1-runway

Revisit fix for #169 - just don't treat 'NA' as a nil-like value

v0.12.0-runway: Fix Issue #169 - NA is still converted to NaN even in a string column

Patch fix for go-gota#169 - NA is still converted to NaN even in a string column

Copied from go-gota#175