fix(html-template-element): fix template.content.cloneNode

[StrahilKazlachev]

where native support is missing fix aurelia/templating#569

@jdanyow could you review?
I'm not sure how good idea is to reassign .cloneNode for a second time in lazySafeCloneNode.

[EisenbergEffect]

@jdanyow Are you able to review this?

[jdanyow]

@StrahilKazlachev sorry for the delay- this is looking good, I agree with you, no need to use the lazy pattern here.

Would you be willing to refactor the lazy stuff out and add a test?

If you npm install karma-ie-launcher as a dev dependency, karma start --browsers IE will run the test suite against IE. This would be a great repo to do IE testing in... we do the same in binding.

[StrahilKazlachev]

@jdanyow Will refactor and ping you when ready.

[StrahilKazlachev]

@jdanyow So it took me a lot more time to come back to this 😭
Finding and caching child templates is deferred till first call to cloneNode(true) - we don't want to miss any child templates added after the parent was created - the second test for ensureHTMLTemplateElement should cover this scenario.

[EisenbergEffect]

@jdanyow What do you think? Good to merge?

[EisenbergEffect]

Ping @jdanyow for review.

[EisenbergEffect]

@jdanyow Could you take another look at this?

[EisenbergEffect]

@StrahilKazlachev Do you feel pretty good about this?