[decorators] Single quotes in the contentText make the content not being rendered at all

- VSCode Version: 1.9.0
- OS Version: OSX

Steps to Reproduce:

1. Before 1.9.0, a code like
```javascript
decoration.renderOptions = {after: {contentText: "'a'" }};
```
rendered `'a'` decoration text. In 1.9.0, nothing is rendered.
