Add integer attribute type.

Merges some common code between integer and enum.
Enum can now bw a set.
Add valuer must take a interface{} in addValue since it can be different types.

packages/ipp/mimemediatype.go

@@ -31,6 +31,6 @@ func (m *mimeMediaType) marshal() []byte {
 	return m.sos.marshal()
 }
 
-func (m *mimeMediaType) addValue(v string) {
-	m.sos.AddValue(v)
+func (m *mimeMediaType) addValue(v interface{}) {
+	m.sos.AddValue(v.(string))
 }