When using `lang="pug"`, passing Boolean `true` as prop and using directives will case errors.
See:
https://github.com/vuejs/vue-loader/issues/693https://github.com/vuejs/vue-loader/issues/55
For example:
```html
<template lang="pug">
foo(
bar
v-baz-directive
)
</template>
```
This will be rendered as
`<foo bar="bar" v-baz-directive="v-baz-directive">`
and cause errors such as `bar expected Boolean but got String` and `v`/`baz`/`directive` not defined.
