getExternalDependencies used in includesComplete test limited to js and css files

Document->getExternalDependencies so far took all urls that belong to <link> or <script> tags, which includes things like canonical, alternate (for rss an so on) urls. The page behavior is not really dependent on alternates and canonicals.
It seems to me this was meant to check if js and css are available, which the function now does.

Also provided unit test with simple fixture for Document.