Retry page.click on relative link

Some erroneous web pages at contain links like `<a title="What is SEO?" href="vocabulary/seo">SEO</a>`with relative **href**.

Assume tihs link is placed at `http://some.domain.com/vocabulary`. That means that mechanize will tries `http://some.domain.com/vocabulary/vocabulary/seo` which ends up with 404 error.

Browsers use a different approach - they treat such href as absolute in case relative url fails.
That patch implements described approach.