Fix TCP checksum calculation.
Fix TCP checksum calculation.

Fix for Issue #4.

1d4Nf6/gopacket,LrsK/gopacket,clarktrimble/gopacket,zerklabs/gopacket,wheelcomplex/gopacket,LrsK/gopacket,clarktrimble/gopacket,levigross/gopacket,dazer-chen/gopacket,dutchcoders/gopacket,packetb-old/gopacket,adriantam/gopacket,urso/gopacket,ccordes-snaproute/gopacket,breml/gopacket,xuzhaokui/gopacket,packetb-old/gopacket,nhooyr/gopacket,ross-williams/gopacket,1d4Nf6/gopacket,dutchcoders/gopacket,google/gopacket,scroot/gopacket,cmillward/gopacket,josephcopenhaver/gopacket,ross-williams/gopacket,aeppert/gopacket,packetbeat/gopacket,packetbeat/gopacket,UIKit0/gopacket-1,lokikiller/gopacket,hkwi/gopacket,cmillward/gopacket,aeppert/gopacket,hkwi/gopacket,lokikiller/gopacket,packetbeat/gopacket,vtolstov/gopacket,aeppert/gopacket,xuzhaokui/gopacket,scroot/gopacket,dutchcoders/gopacket,ccordes-snaproute/gopacket,rouzier/gopacket,UIKit0/gopacket-1,cmillward/gopacket,chapayGhub/gopacket,nhooyr/gopacket,breml/gopacket,rouzier/gopacket,skyportsystems/gopacket,xuzhaokui/gopacket,dazer-chen/gopacket,wheelcomplex/gopacket,breml/gopacket,google/gopacket,adriantam/gopacket,scroot/gopacket,levigross/gopacket,skyportsystems/gopacket,zerklabs/gopacket,ross-williams/gopacket,google/gopacket,adriantam/gopacket,josephcopenhaver/gopacket,skyportsystems/gopacket,UIKit0/gopacket-1,vtolstov/gopacket,chapayGhub/gopacket,LrsK/gopacket,packetbeat/gopacket,lokikiller/gopacket,josephcopenhaver/gopacket,zerklabs/gopacket,rouzier/gopacket,urso/gopacket,hkwi/gopacket,urso/gopacket,nhooyr/gopacket,dazer-chen/gopacket,wheelcomplex/gopacket,packetb-old/gopacket,clarktrimble/gopacket,levigross/gopacket,vtolstov/gopacket,chapayGhub/gopacket,1d4Nf6/gopacket,packetb-old/gopacket,ccordes-snaproute/gopacket