Add conversion from enum_<> to class_<>

Author: aldanor

include/pybind11/pybind11.h

@@ -1261,6 +1261,10 @@ template <typename Type> class enum_ {
         return cls;
     }
 
+    operator class_<Type>() && {
+        return into_class();
+    }
+
 private:
     class_<Type> cls;
     dict m_entries;