Fixing a ConcurrentModificationException in JsonRequestProcessor that occurs when you use a setter that sets an Entity from null to a value. Also fixes a bug in DeltaValueStoreJsonImpl#retainValue() where we only retain the first element of a List instead of all elements in the list because the return statement is in the wrong place.  This revealed other NPEs on the server that are also fixed.

Review at http://gwt-code-reviews.appspot.com/945801

Review by: robertvawter@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@8920 8db76d5a-ed1c-0410-87a9-c151d255dfc7