added null guard in CachingHandler
added null guard in CachingHandler

modulexcite/WebAPIContrib,TerraVenil/WebAPIContrib,yonglehou/WebAPIContrib,modulexcite/WebAPIContrib,yonglehou/WebAPIContrib,andyshao/WebAPIContrib,WebApiContrib/WebAPIContrib,WebApiContrib/WebAPIContrib,andyshao/WebAPIContrib,TerraVenil/WebAPIContrib