Fix handling of "default" typemap in Python.

Use "compact" arguments form for the function if "default" typemap is defined
for any of its arguments to allow omitting this argument when calling it from
Python.

Closes #377.

Examples/test-suite/python/default_args_runme.py

@@ -43,6 +43,12 @@ def run(module_name):
   if f.double_if_handle_is_null(5) != 10:
     raise RuntimeError
 
+  if f.double_if_dbl_ptr_is_null(6, None) != 12:
+    raise RuntimeError
+
+  if f.double_if_dbl_ptr_is_null(7) != 14:
+    raise RuntimeError
+
   try:
     f = default_args.Foo(1)
     error = 1