Make the FailureEndpoint respect the strategy's path_prefix.

Previously, it just fetched the global path_prefix.

lib/omniauth/failure_endpoint.rb

@@ -27,10 +27,18 @@ module OmniAuth
 
     def redirect_to_failure
       message_key = env['omniauth.error.type']
-      new_path = "#{env['SCRIPT_NAME']}#{OmniAuth.config.path_prefix}/failure?message=#{message_key}#{origin_query_param}#{strategy_name_query_param}"
+      new_path = "#{env['SCRIPT_NAME']}#{strategy_path_prefix}/failure?message=#{message_key}#{origin_query_param}#{strategy_name_query_param}"
       Rack::Response.new(['302 Moved'], 302, 'Location' => new_path).finish
     end
 
+    def strategy_path_prefix
+      if env['omniauth.error.strategy']
+        env['omniauth.error.strategy'].path_prefix
+      else
+        OmniAuth.config.path_prefix
+      end
+    end
+
     def strategy_name_query_param
       return '' unless env['omniauth.error.strategy']
 

spec/omniauth/failure_endpoint_spec.rb

@@ -43,12 +43,18 @@ describe OmniAuth::FailureEndpoint do
       expect(head['Location']).to eq('/random/auth/failure?message=invalid_request&strategy=test')
     end
 
-    it 'respects the configured path prefix' do
+    it 'respects the globally configured path prefix' do
       allow(OmniAuth.config).to receive(:path_prefix).and_return('/boo')
       _, head, = *subject.call(env)
       expect(head['Location']).to eq('/boo/failure?message=invalid_request&strategy=test')
     end
 
+    it 'respects the custom path prefix configured on the strategy' do
+      env['omniauth.error.strategy'] = ExampleStrategy.new({}, path_prefix: "/some/custom/path")
+      _, head, = *subject.call(env)
+      expect(head['Location']).to eq('/some/custom/path/failure?message=invalid_request&strategy=test')
+    end
+
     it 'includes the origin (escaped) if one is provided' do
       env['omniauth.origin'] = '/origin-example'
       _, head, = *subject.call(env)