* In Article::replaceSection(), actually return null when $section is bogus. Used...

* In Article::replaceSection(), actually return null when $section is bogus. Used this in my half-complete and now kind of abandoned attempt at rewriting EditPage.php
* In UserMailer::send(), allow the caller to override the Content-type header so people can send multipart messages if they really want to. Using this in an extension I'm developing