all members optional be default in struct

this apllies only to the argument in frappy - in the case
of all members optional, they are put into the description,
not honouring issue 69

Change-Id: I8e9e4d9ef6bd5b9bb2748f3c6116b7094cd9e927

secop/datatypes.py

@@ -849,7 +849,7 @@ class ImmutableDict(dict):
 class StructOf(DataType):
     """data structure with named fields
 
-    :param optional: a list of optional members
+    :param optional: a list of optional members (default None: all members optional)
     :param members: names as keys and types as values for all members
     """
     def __init__(self, optional=None, **members):
@@ -857,7 +857,7 @@ class StructOf(DataType):
         self.members = members
         if not members:
             raise BadValueError('Empty structs are not allowed!')
-        self.optional = list(optional or [])
+        self.optional = list(members) if optional is None else list(optional)
         for name, subtype in list(members.items()):
             if not isinstance(subtype, DataType):
                 raise ProgrammingError(