fixing abstract method inheritance issue

TypeNodes.py

@@ -156,7 +156,7 @@ class ClassNode(ClassInterNode):
 
         # 10. A class that contains (declares or inherits) any abstract methods must be abstract.
         for con in contains:
-            if 'abstract' in con.mods and 'abstract' not in self.mods:
+            if ('abstract' in con.mods or not con.body) and 'abstract' not in self.mods:
                 raise Exception("ERROR: Non-abstract Class '{}' contains an abstract method".format(self.name))
 
     # hierarchy: string[]