std.parseYaml processes "on" as a boolean

I noticed this on go-jsonnet but couldn't replicate with the C or rust version:

```
➜ cat a.yaml
on: help

➜ jsonnet -e 'std.parseYaml(importstr "a.yaml")'
{
"true": "help"
}

➜ cjsonnet -e 'std.parseYaml(importstr "a.yaml")'
{
"on": "help"
}

➜ jrsonnet -e 'std.parseYaml(importstr "a.yaml")'
{
"on": "help"
}
```