GRAILS-11333: Handle already absolute URLs

When a "uri" is provided to the link method and the "absolute" parameter is true, check if the provided uri is already absolute before trying to make it absolute.

Note that this pull request is fixed and doesn't have any embarrassingly bad problems like grails/grails-core/pull/488 had. I ran "gradlew test" in "grails-web" and all tests pass.