message: Fix passing of sd_bus interface to new messages

This breaks unit testing if we try and mock out the creation of method
replies and errors since it will try and use the default implementation
instead of the one provided to the initial message or bus.

Change-Id: I700bef0bae9a8a371ec0121f8115acd30aee5220
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/sdbusplus/message.hpp b/sdbusplus/message.hpp
index 11cfb9f..a307a6f 100644
--- a/sdbusplus/message.hpp
+++ b/sdbusplus/message.hpp
@@ -300,7 +300,7 @@
             throw exception::SdBusError(-r, "sd_bus_message_new_method_return");
         }
 
-        return message(reply, std::false_type());
+        return message(reply, _intf, std::false_type());
     }
 
     /** @brief Create a 'method_error' type message from an existing message.
@@ -318,7 +318,7 @@
             throw exception::SdBusError(-r, "sd_bus_message_new_method_errno");
         }
 
-        return message(reply, std::false_type());
+        return message(reply, _intf, std::false_type());
     }
 
     /** @brief Perform a 'method-return' response call. */