specs: Use StdMethod accurately.
diff --git a/specs/stdapi.py b/specs/stdapi.py
index 8a5a5a7..846227e 100644
--- a/specs/stdapi.py
+++ b/specs/stdapi.py
@@ -460,7 +460,8 @@
 
 class Method(Function):
 
-    def __init__(self, type, name, args, call = '__stdcall', const=False, sideeffects=True):
+    def __init__(self, type, name, args, call = '', const=False, sideeffects=True):
+        assert call == '__stdcall'
         Function.__init__(self, type, name, args, call = call, sideeffects=sideeffects)
         for index in range(len(self.args)):
             self.args[index].index = index + 1