Canonical fix

When you are using this plugin with a page that doesn't contain a data.path attribute it results in 

`http://canonical.com/undefined/`

I added a simple check for data.path and added it accordingly. I also removed the trailing slashes, no need for them when it is dynamically building the url.

Resulting pages are now.. 

Collection-Pagination - `http://canonical.com/category/foo/index.html`
Index - `http://canonical.com`
Single Post - `http://canonical.com/post/foo`

Thanks...