Fix some remarkup issues with selective construction of child nodes after HTML DOM parsing

Summary:
See https://discourse.phabricator-community.org/t/bug-in-src-parser-html-phutildomnode-php/2882/.

The behavior of selectChildrenWithTags(...) currently isn't well-covered by tests and isn't quite right. Add coverage and fix it up.

Particularly, there's a bad call to a nonexistent method, and when we convert a tag that doesn't match the selection set into content, we incorrectly discard any paired closing tag.

Test Plan: Added failing tests, made them pass.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20630