Fixed the loading of link element source

[tomvanenckevort]

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Description

This is a fix for #1184

The issue was that the _relation variable was only set during the SetupElement method. But when adding a new link element the rel attribute is often not set at this point. So if rel and href are set after creation it will never call _relation?.LoadAsync(), and therefore the resource loader won't kick in.

Changes:

• Updated HtmlLinkElement to make sure that it attempts to load the relation when either the rel, href attributes have changed, or when the parent has changed (i.e. when it's added to the document). I took inspiration from the HtmlScriptElement where something similar happens.
• Updated DefaultAttributeObserver to trigger the new UpdateRel method when the rel attribute changes.
• Added a new unit test to prove that newly added links now trigger the right resource loader.

[CLAassistant]

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[FlorianRappl]

LGTM - thanks!