convertNode handles escaping text improperly

First reported  in https://github.com/sourcegraph/browser-extensions/issues/139. Added a failing test case [here](https://sourcegraph.com/github.com/sourcegraph/codeintellify@convert-node-changes-text/-/blob/src/token_position.test.ts#L81-89).

![screen shot 2018-09-06 at 10 20 37 am](https://user-images.githubusercontent.com/11557758/45174597-248f2c00-b1c0-11e8-8a8b-9f52011c6774.png)
