Stringify incoming group ids

Otherwise when we try to make their local storage keys they look like:
"egroupByteBuffer(offset=4,markedOffset=-1,length=28,capacity=112)"

js/helpers.js

@@ -646,6 +646,7 @@ window.textsecure.subscribeToPush = function(message_callback) {
 				var promises = [];
 
 				if (decrypted.group !== null) {
+                    decrypted.group.id = getString(decrypted.group.id);
 					var existingGroup = textsecure.storage.groups.getNumbers(decrypted.group.id);
 					if (existingGroup === undefined) {
 						if (decrypted.group.type != textsecure.protos.PushMessageContentProtobuf.GroupContext.Type.UPDATE)