Add a describe block for the public file with content

Also add a comment about why it's hard to check for nil because of Rack.

spec/riak_spec.rb

@@ -34,6 +34,7 @@ describe "App with Riak backend" do
   end
 
   describe "GET public data" do
+    describe "file with content" do
       before do
         object = data_bucket.new("jimmy:public:foo")
         object.content_type = "text/plain"
@@ -62,6 +63,7 @@ describe "App with Riak backend" do
         last_response.status.must_equal 200
         last_response.headers["Expires"].must_equal "0"
       end
+    end
 
     describe "empty file" do
       before do
@@ -75,7 +77,11 @@ describe "App with Riak backend" do
 
       it "returns an empty body" do
         last_response.status.must_equal 200
-        last_response.body.must_equal ""
+        # Rack::MockRequest turns the body into a string. We can't use
+        # `last_response.body` to check for nil, because:
+        # >> [nil].join
+        # => ""
+        last_response.body.must_equal ''
         last_response.headers["Content-Length"].must_equal '0'
       end
     end