Using text/html in spec instead of text/plain

text/plain throws an unknown content type error rails/rails#3234

spec/controllers/cangaroo/endpoint_controller_spec.rb

@@ -20,7 +20,7 @@ module Cangaroo
       it 'accepts only application/json requests' do
         expect(response.status).to eq(202)
 
-        request.headers['Content-Type'] = 'application/text'
+        request.headers['Content-Type'] = 'text/html'
         post :create
         expect(response.status).to eq(406)
       end