Remove empty per_page query string from first page URL.

In response to #2124. The Pagination library adds the `per_page` query string to the `base_url`, but the first page is empty, resulting in a URL like `example.com/welcome?per_page=`. This fix assigns the `base_url` to the `first_url` option, if a custom one does not already exist.

I'm not sure if this fix will affect the `reuse_query_string` feature, since I've never used it. Pretty sure it won't, but if anyone can test and verify, that'd be swell.

Signed-off-by: Eric Roberts <eric@cryode.com>