Fix wrong #error method usage

Author: mizoR

examples/echobot/app.rb

@@ -13,7 +13,7 @@ def client
 
   signature = request.env['HTTP_X_LINE_SIGNATURE']
   unless client.validate_signature(body, signature)
-    error 400 do 'Bad Request' end
+    halt 400, {'Content-Type' => 'text/plain'}, 'Bad Request'
   end
 
   events = client.parse_events_from(body)

examples/kitchensink/app.rb

@@ -40,7 +40,7 @@ def reply_content(event, messages)
 
   signature = request.env['HTTP_X_LINE_SIGNATURE']
   unless client.validate_signature(body, signature)
-    error 400 do 'Bad Request' end
+    halt 400, {'Content-Type' => 'text/plain'}, 'Bad Request'
   end
 
   events = client.parse_events_from(body)