Ah, nice catch.  I'll add the header in when I print the error.  Should I keep the response body the way it is, or remove it?  I think it might be nice to keep the body, but I'm not convinced everyone would want that as the default behavior.

In terms of implementation, I'll also store the Allow header data in a new context variable.  That context variable will be specific to the missing header errors, so it won't be used for any other error.