Two faults found on review

A second Accept value would have overwritten the first, because the header was Set per value rather
than Added. One value is all any caller passes today, so nothing was wrong — but a helper that
quietly keeps only the last of what it was given is a trap for whoever passes two.

And a replayed announcement that could not be written was published as an empty body: a fact on the
mesh that says nothing, which the reader can only log and drop. It is now said and skipped, because a
body that cannot be marshalled is this program's fault rather than the bus's.

internal/builder/registry.go

@@ -137,7 +137,7 @@ func (r Registry) has(ctx context.Context, url string, accept ...string) (bool,
 		return false, err
 	}
 	for _, media := range accept {
-		request.Header.Set("Accept", media)
+		request.Header.Add("Accept", media)
 	}
 	response, err := r.client().Do(request)
 	if err != nil {