fixed .Net 4.5 build
fixed .Net 4.5 build

git-tfs-id: [https://tfs.codeplex.com/tfs/TFS05/]$/ZXingNet/trunk;C84051

bsoja/ZXing.Net,micjahn/ZXing.Net,micjahn/ZXing.Net,DHMechatronicAG/ZXing.Net,DHMechatronicAG/ZXing.Net,DHMechatronicAG/ZXing.Net,bsoja/ZXing.Net,micjahn/ZXing.Net,bsoja/ZXing.Net