_.escape incorrectly escapes backticks

``` js
var _ = require('lodash');

console.log(_.escape('`test`'))
// > "undefinedtestundefined"
```

Online example: https://runkit.com/577a60557da47e130044a1c5/57e142c26eefb414003d332e
