BP_Group_Extension::enable_nav_item should default to true

This is in keeping with backward compatibility requirements for 1.7-era plugins

It also ensures that the default behavior is the most common behavior, ie the
new nav item should be available.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7273 cdf35c40-ae34-48e0-9cc9-0c9da1808c22