[BUGFIX] Don't call toString at extend time

Doing so will memoize the function so that any Ember.View subclass will return the same value for toString.

This can be seen here: http://emberjs.jsbin.com/UsENAKiL/1/edit
It was introduced by c2a83893f5df634c0037b8a194c78734ba21ef5f

Probably a better way to fix this is to make sure that a view does not get the memoized toString from the prototype, I just wanted a quick fix before 1.3.0 is released.