Fix to enable better error message on middleware.insert_before

Patch against Rails 3.0.pre (edge)

Without the patch, if you try this:

...middleware.insert_before(
  Non::Existent::Middleware,
  My::Middleware
)

You get:

Exiting /Users/nwiger/Workspace/op/proto/fbapp/vendor/gems/dirs/rails/actionpack/lib/action_dispatch/middleware/stack.rb:90:in insert': no implicit conversion from nil to integer (TypeError)

from /Users/nwiger/Workspace/op/proto/fbapp/vendor/gems/dirs/rails/actionpack/lib/action_dispatch/middleware/stack.rb:90:in `insert'

But insert_after already has better handling, so I just adapted that to insert_before. So with the patch, you now get:

Exiting

/Users/nwiger/Workspace/op/proto/fbapp/vendor/gems/dirs/rails/actionpack/lib/action_dispatch/middleware/stack.rb:89:in insert': No such middleware to insert before: Non::Existent::Middleware (RuntimeError)