Call replace on string instead of {string.js} object#184

To reproduce: `test.js` ``` javascript // Invoke section of babel-polyfill code that resets Object.prototype import 'babel-polyfill'; for (let _ of []) { _; } import S from 'string'; S('Test').slugify().s; ``` ``` bash $ nvm use v0.10.24 $ npm install babel-cli babel-preset-es2015 babel-polyfill string $ node_modules/.bin/babel --presets es2015 test.js > test.es5.js $ node test.es5.js /git/test/node_modules/string/lib/string.js:134 var s = this.replace(/[^A-Za-z0-9\[\] ]/g, function(x) { return latin_ma ^ TypeError: Object Test has no method 'replace' at S.latinise (/git/test/node_modules/string/lib/string.js:134:20) at S.slugify (/git/test/node_modules/string/lib/string.js:413:37) at Object.<anonymous> (/git/test/test.es5.js:20:31) at Module._compile (module.js:456:26) at Object.Module._extensions..js (module.js:474:10) at Module.load (module.js:356:32) at Function.Module._load (module.js:312:12) at Function.Module.runMain (module.js:497:10) at startup (node.js:119:16) at node.js:902:3 ```