Add a test for "link rel='next'" kind of link.

While this is not strictly valid, we don't put the link markup into
`<head>` to avoid adding more code just for those two new tests.
Match mixed-case relationship links in goNext/goPrevious

Link types are case-insensitive:
 http://www.w3.org/TR/html5/links.html#linkTypes

This should fix #1115.
Merge pull request #1116 from vincentbernat/fix/link-gonext

Match mixed-case relationship links in goNext/goPrevious