Strong Params + Refactor

1. New private method ```doc_params``` to sanitize ```title```, ```content```, and ```tags[]```.
2. New private method ```set_doc```. Refactored to replace ```@doc = Doc.find(params[:id])```.
3. New ```before_action``` which calls ```set_doc``` for ```[:show, :edit, :update, :destroy]``` methods