Wrongly interprets unicode strings

# How to reproduce
```
# in.tmpl
{{ env ("A") | default "[^\u0000-\uFFFF]" }}
```
```
$ goenvtemplator2 -version
2017/10/12 15:03:02 Version: -2.0.0-rc3
$ goenvtemplator2 -template $PWD/in.tmpl:/dev/stdout
# in.tmpl
[^-￿]
```