Resolve test name conflict

There are two tests named
'sets the Content-Type response header without extname' in
helpers_test.rb.

Rename the second to accurately describe what it tests.

test/helpers_test.rb

@@ -601,7 +601,7 @@ class HelpersTest < Test::Unit::TestCase
       assert_equal '<sinatra></sinatra>', body   
     end
     
-    it 'sets the Content-Type response header without extname' do
+    it 'sets the Content-Type response header with extname' do
       mock_app do
         get('/attachment') do
           content_type :atom