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/request.go

@@ -33,7 +33,7 @@ func (h ippMessageHeader) String() string {
 }
 
 type AddValuer interface {
-	addValue(string)
+	addValue(interface{})
 }
 
 type Request struct {