FIX: check for nil response on JsonCsrf protection
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.

ryanshaw/sinatra,JonMidhir/sinatra,burningTyger/sinatra,JonMidhir/sinatra,jkowens/sinatra,jkowens/sinatra,mwpastore/sinatra,sinatra/sinatra,sinatra/sinatra,burningTyger/sinatra,JonMidhir/sinatra,mwpastore/sinatra,jkowens/sinatra,burningTyger/sinatra,ryanshaw/sinatra,mwpastore/sinatra