#176 Group parsing problem

Group gets corrupted when parsing message from string and MessageFactory
is not set or MessageFactory fails to create typed Group. Parsing fails
on the second instance of the group.

Grp must be reset to null when the last instance of group was added to
fieldMap.

Message.SetGroup method:
...
// Create a new group!
if (msgFactory != null)
grp = msgFactory.Create(Message.ExtractBeginString(msgstr),
Message.GetMsgType(msgstr), grpNoFld.Tag);

//If above failed (shouldn't ever happen), just use a generic Group.

//********** grp already contains first instance of group.
if (grp == null)
grp = new Group(grpNoFld.Tag, grpEntryDelimiterTag, order);