Resolve test name conflict

In delegator_test.rb, there are two tests named
"registers helpers with the delegation target", causing "method
redefined" warnings and the first test not to be run.

Rename the second test so that all tests run.

test/delegator_test.rb

@@ -100,7 +100,7 @@ def target
     assert_equal ["helpers", mixin.to_s], app.last_call
   end
 
-  it "registers helpers with the delegation target" do
+  it "registers middleware with the delegation target" do
     app, mixin = mirror, Module.new
     Sinatra.use mixin
     assert_equal ["use", mixin.to_s], app.last_call