Add `fullBase` option to `HtmlHelper::link()`

Contrary to the other helper methods, `link()` was only accepting an option named `full_base` that had to be passed in the URL array.

I think having this functionality to work uniformly across the helper would be nice, though it seems to be still the same in 3.x, so I'm not sure whether this is considered to be useful, maybe I've overlooked some drawbacks.

Note that the last test uses `assertEquals()` because the `/posts` content of the `a` tag seems to break `assertTags()`, not sure whether this is a bug or just not supported.
I thought that when explicitly using it in the options, it should take precedence over `full_base`, which would not be the case by default as the latter overrides the `$full` argument it `Router::url()`.