Added more error handling and necessary functions

id.go

@@ -9,16 +9,16 @@ import (
 type ID struct {
 	XMLName xml.Name `xml:"id"`
 	*CommonAttributes
-	URI IRI `xml:",chardata"`
+	URI string `xml:",chardata"` // IRI
 }
 
 // NewID creates a new ID. It returns a *ID and an error.
-func NewID(uri IRI) (*ID, error) {
+func NewID(uri string) (*ID, error) {
 	if !isValidIRI(uri) {
 		return nil, fmt.Errorf("uri %v not correctly formatted", uri)
 	}
 
-	return &ID{URI: IRI(uri)}, nil
+	return &ID{URI: uri}, nil
 }
 
 // Check checks the ID for incompatibilities with RFC4287. It returns an error.