How to disable the default WordPress redirect to non-www URLs?

Question

I am trying to redirect all non-www to www URLs in my WordPress MultiSite install. I am using a simple .htaccess rule like this:

# Redirect non-www to www:
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]

Normally this would work, however WordPress already seems to have a default redirection to non-www URLs. This means that when I use that RewriteRule it causes an infinite redirect loop.

How do I disable this?

PS. I tried define('NOBLOGREDIRECT', 'http://www.domain.com'); in wp-config.php with no effect.