Don't throw an exception when getting/setting 'href' for relation types that it doesn't make sense for.

Just allow it to be set as a regular property so it's effectively a "don't care". Prevents a brutal failure when someone goes:

assetGraph.findRelations({...}, function (relation) {
    relation.hrefType = 'absolute';
});

... without checking the type of each relation instance.

(cherry picked from commit b9600fcde66555972e04a0b82ccdf204fd8cee89)

Conflicts:
	lib/relations/HtmlIFrameSrcDoc.js
	lib/relations/HtmlImageSrcSet.js
	lib/relations/HtmlPictureSourceSrcSet.js
1.5.13