Stop params[:form][:file] in test_app from crashing

This got triggered on Travis -- I am not sure what is different in their
environment.

I prefer this solution over (params[:form] && params[:form][:file])
so the test doesn't silently pass if something goes badly wrong

lib/capybara/spec/views/form.erb

@@ -274,6 +274,7 @@
   </p>
 
   <p>
+    <input type="hidden" name="form[dummy]" value="ensure params[:form] exists"/>
     <input type="submit" value="Upload Empty"/>
   <p>
 </form>