Fixes media tests and updates how the IPublishedContent is created from the xpath iterator
Fixes build and fixes how published media cache gets root nodes - if people have done this in the past it would have been horrible for site performance
removes a MoveNext() call since we're no longer dealing with xdocuments and instead xelements
Merge branch '7.1.9' into 7.2.0

Conflicts:
	src/Umbraco.Tests/PublishedContent/PublishedMediaTests.cs
fix merge