Why no <meta charset="utf-8" /> in application.html.erb?

I'm reading "Dive into html5" and in http://diveintohtml5.org/semantics.html#encoding
, it says every page should contain <meta charset="utf-8" />,
otherwise it's subject to security vulnerabilities (http://
code.google.com/p/doctype/wiki/ArticleUtf7). I don't see <meta
charset="utf-8" /> being generated in my application.html.erb (in
rails3) Is there a reason behind this?

because rails sets the content-type header (which, as the article you
linked to notes, is the preferred mechanism for this)