No more undefined content in link and category

It seems that undefined content is only mentioned in RFC4287 because
link and category are elements and those usually need some content.

link.go

@@ -9,7 +9,6 @@ import (
 type Link struct {
 	*CommonAttributes
 	Title    string      `xml:"title,attr,omitempty"`
-	Content  string      `xml:",chardata"` // undefinedContent in RFC4287
 	Href     IRI         `xml:"href,attr"`
 	Rel      string      `xml:"rel,attr,omitempty"`
 	Type     MediaType   `xml:"type,attr,omitempty"`
@@ -18,14 +17,8 @@ type Link struct {
 }
 
 // NewLink creates a new Link. It returns a *Link and an error.
-func NewLink(href, content string) (*Link, error) {
-	if content != "" {
-		if !isValidXML(content) {
-			return nil, fmt.Errorf("%v not valid XML", content)
-		}
-	}
-
-	return &Link{Href: IRI(href), Content: content}, nil
+func NewLink(href, content string) *Link {
+	return &Link{Href: IRI(href)}
 }
 
 // Check checks the Link for incompatibilities with RFC4287. It returns an
@@ -57,12 +50,6 @@ func (l *Link) Check() error {
 		}
 	}
 
-	if l.Content != "" {
-		if !isValidXML(l.Content) {
-			return fmt.Errorf("content element %v of link not valid XML", l.Content)
-		}
-	}
-
 	return nil
 }