Adding documentation and tests to ``polymorphic_url`` and ``link_to``

Right now, it's hard to find any documentation to tell you if its possible to do this:

```ruby
polymorphic_url([:comments], anchor: "comment_#{comment.id}")
```

However it is possible because ``polymorphic_url`` sends its options as arguments to ``url_for``. This patch documents the options and adds tests to make sure the options work.