NA is still converted to NaN even in a string column

[indeedhat]

Issue

It doesn't seem to matter what the column type is in the data frame NA will always get converted to NaN.

Expected Behavior

NA would only be converted to NaN in int and float columns

Actual Behaviour

NA is converted to NaN regardless of the type

Example

Take this modified version of some data from the README

package main

import (
	"fmt"
	"strings"

	"github.com/go-gota/gota/dataframe"
	"github.com/go-gota/gota/series"
)

const ExampleData = `
Country,Region,Date,Age,Amount,Id
"United States",US,2012-02-01,50,112.1,01234
"United States",US,2012-02-01,32,321.31,54320
"United Kingdom",GB,2012-02-01,17,18.2,12345
"United States",NA,2012-02-01,32,321.31,54320
"United States","NA",2012-02-01,17,321.31,54320
"United Kingdom",GB,2012-02-01,NA,18.2,12345
"United States",NA,2012-02-01,32,321.31,54320
Spain,EU,2012-02-01,66,555.42,00241
`

func main() {
    frame := dataframe.ReadCSV(
        strings.NewReader(ExampleData),
        dataframe.WithTypes(map[string]series.Type{
            "Age": series.String,
        }),
    )

    fmt.Println(frame)
}

which produces the following output

[8x6] DataFrame

    Country        Region   Date       Age      Amount     Id
 0: United States  US       2012-02-01 50       112.100000 1234
 1: United States  US       2012-02-01 32       321.310000 54320
 2: United Kingdom GB       2012-02-01 17       18.200000  12345
 3: United States  NaN      2012-02-01 32       321.310000 54320
 4: United States  NaN      2012-02-01 17       321.310000 54320
 5: United Kingdom GB       2012-02-01 NaN      18.200000  12345
 6: United States  NaN      2012-02-01 32       321.310000 54320
 7: Spain          EU       2012-02-01 66       555.420000 241
    <string>       <string> <string>   <string> <float>    <int>

In both colums:

• Region with an implicit string type
• Age where i have explicitly set the type to string

The string value NA is converted

[prliu]

This problem is due to the ReadCSV function converting the "NA" string to "NaN" before we assign a data type to the column, it should be tagged as a bug.

I'm trying to make a patch for this issue.

[indeedhat]

Much appreciated, i look forward to this being merged :)