Applied patch from jira issue - OFBIZ-5416 - Sales order cart items already assigned to ship groups are incorrectly able to be modified to quantities/prices that break the ProductStoreShipmentMeth rules.
Thanks Christian for reporting the issue and Thanks Arun for providing the patch. 

git-svn-id: https://svn.apache.org/repos/asf/ofbiz/trunk@1639835 13f79535-47bb-0310-9956-ffa450edef68