Further fixes

As per the change for #20, handle the case where the user passes in
a `defaults` kwarg to `pysipp.scenario` and use it override the normal
template. To fully solve #12 also support assignment of `None` which
results in the tuple attribute taking a value of `(None, None)`
implicitly.

pysipp/agent.py

@@ -24,8 +24,11 @@ def getter(self):
 
     def setter(self, pair):
         if not isinstance(pair, tuple):
-            raise ValueError("{} must be a tuple".format(pair))
-        for attr, val in zip(attrs, pair or itertools.repeat(None)):
+            if pair is None:
+                pair = (None, None)
+            else:
+                raise ValueError("{} must be a tuple".format(pair))
+        for attr, val in zip(attrs, pair):
             setattr(self, attr, val)
 
     doc = "{} parameters composed as a tuple".format(', '.join(attrs))
@@ -233,6 +236,12 @@ def Scenario(agents, **kwargs):
         if key in _defs:
             _defs[key] = kwargs.pop(key)
 
+    # if a `defaults` kwarg is passed in by the user override template values with
+    # values from that as well
+    user_defaults = kwargs.pop('defaults', None)
+    if user_defaults:
+        _defs.update(user_defaults)
+
     # this gives us scen.<param> attribute access to scen.defaults
     utils.DictProxy(_defs, UserAgent.keys(), cls=scentype)
     return scentype(agents, _defs, **kwargs)