Enabling injectJs + a test

Hey, I have absolutely no idea what's going on. Looking at https://github.com/sheebz/phantom-proxy/commit/3cb262b0e394093d4fd03142cdcfb39da088121a it appears you already merged this commit into master, but `lib/webpage.js` does not have the changes in it. So injectJS is not working. This re-adds it?