fix(tXml): html unescape node attributes with urls

[dave-nicholas]

To provide feature parity with the DOMParser.getAttribute API which would unescape html entities for free this PR unescapes url strings which come from node attributes.

In a similar vain to #6198 which unescapes child strings.

[joeyparrish]

The other PR involved a smaller change, in which the parsing change was inside Txml instead of at every call site. I think I would prefer this to go into Txml, too, unless there is some good reason to do otherwise.

[shaka-bot]

Incremental code coverage: 100.00%

[dave-nicholas]

The reason not to do that in this case is to save calling the unescape function whilst parsing every attribute when it is only needed in a few places. The change in the other PR was possible because I had already made a change at every callsite from element.textContent to TXml.getContents(element) in the initial PR. I thought about making a similar method in the TXml library but that method would just call StringUtils.htmlUnescape anyway.

[absidue]

As this custom XML parser is exposed to API users via the dash manifest preprocessor option, things that happened automatically before but don't now, should probably be documented somewhere. As that will make it easier for developers to migrate their projects to the new XML parser.

[dave-nicholas]

@absidue that is a fair point and I have updated the upgrade documentation to reflect this.

[joeyparrish]

You've convinced me on your approach. If we can agree on the docs, I'll be happy to merge it. Thanks for your contribution!