Add regression test for issue #50

Running specs you get

Failures:

  1) Rack::Protection::JsonCsrf with drop_session as default reaction reset the session
     Failure/Error: get('/', {}, 'HTTP_REFERER' => 'http://evil.com', 'rack.session' => session)
     NoMethodError:
       undefined method `detect' for nil:NilClass
     # ./lib/rack/protection/base.rb:107:in `html?'
     # ./lib/rack/protection/frame_options.rb:32:in `call'
     # ./spec/json_csrf_spec.rb:54:in `block (3 levels) in <top (required)>'
FIX: check for nil response on JsonCsrf protection

Some reaction do not return a response, think for example drop_session. In that case a nil response
would be returned, see issue #50.
Merge pull request #52 from bugant/master

Check for nil response on JsonCsrf protection