Fix several content type negotiation issues.

* No `Accept:` header should be the same as `Accept: */*`.
* For unresolved reference error, `text/plain` should take priority.

src/http.go

···
 }
 
 func ParseAcceptHeader(headerValue string) (result HTTPContentTypes) {
+	if headerValue == "" {
+		headerValue = "*/*"
+	}
 	result = HTTPContentTypes{parseGenericAcceptHeader(headerValue)}
 	return
 }
···
 		prefs[code] = 0
 	}
 	for _, ctyp := range e.contentTypes {
-		if ctyp.code == "*" || ctyp.code == "*/*" {
+		if ctyp.code == "*/*" {
 			for code := range prefs {
 				prefs[code] = ctyp.qval
 			}

src/pages.go

···
 func reportUpdateResult(w http.ResponseWriter, r *http.Request, result UpdateResult) error {
 	var unresolvedRefErr UnresolvedRefError
 	if result.outcome == UpdateError && errors.As(result.err, &unresolvedRefErr) {
-		offeredContentTypes := []string{"application/vnd.git-pages.unresolved", "text/plain"}
+		offeredContentTypes := []string{"text/plain", "application/vnd.git-pages.unresolved"}
 		acceptedContentTypes := ParseAcceptHeader(r.Header.Get("Accept"))
 		switch acceptedContentTypes.Negotiate(offeredContentTypes...) {
 		default: