Fixed objToHash and Gruntfile

Hi, it's me again:) Seems like I've left unfixed `objToHash` fn. And I've found out why tests in my app were crashing while trying to reproduce them right here gave nothing: the thing is that in `psc` task in Gruntfile option that contains list of modules to optimize is caled `modules` instead of `module` param in psc itself. Thus there were no optimizations for tests and examples in ps-json and all functions were present in all imported modules. But when optimizations are enabled functions that are not directly used are being deleted. In `objToHash` there were `fst` and `snd` which weren't used anywhere else in Data.JSON so they've been optimized out giving nasty runtime errors. I've fixed it too.