make unique id comparison case-insensitive

metadata.go

@@ -65,7 +65,7 @@ func (m *Metadata) Uniques() []string {
 		out = append(out, fmt.Sprintf("isbn:%s", isbn))
 	}
 	// Call it 'sig', but it's just title+author(s).
-	sig := fmt.Sprintf("sig:%s|%s", m.Title, strings.Join(m.Creator, "|"))
+	sig := strings.ToLower(fmt.Sprintf("sig:%s|%s", m.Title, strings.Join(m.Creator, "|")))
 	out = append(out, sig)
 	return out
 }