[Rails] respond_with :json — strange behavior

I have a controller:

class ItemsController < ApplicationController
respond_to :json

def index
groups = Item.all

respond_with(items)
end
end

I am curling *my_app/items.json* and see a response with JSON object — just
as expected. At this point I don't have any views associated with
controller, specifically I don't have *index.html.erb* (*.html*).

Now if only I create an *index.haml* (for instance), with a simple *%h1
Hello, world!* line, requesting (again via curl) *my_app/items.json* returns
an html string with *<h1>Hello, world!</h1>*. Note that I didn't alter the
controller code — it just remains untouched.

I'm sure I'm missing something. Any help on explaining of what's happening
here?