bugfix: convert ints to strings before write

Minor bugfix.

When using "--output text", if the value returned is an integer, this error is returned:

```
# aws ec2 describe-volumes --volume-ids $VOL_ID --query 'Volumes[0].Size' --output json
10
# aws ec2 describe-volumes --volume-ids $VOL_ID --query 'Volumes[0].Size' --output text

argument 1 must be string or read-only character buffer, not int
```

This patch ensures the value is converted to a string before .write.  After patch:

```
# aws ec2 describe-volumes --volume-ids $VOL_ID --query 'Volumes[0].Size' --output text
10
```