Fix source check in entry

entry.go

@@ -34,10 +34,15 @@ type Entry struct {
 // the atom:entry contains an atom:source element that contains an atom:author
 // element or, in an Atom Feed Document, the atom:feed element contains an
 // atom:author element itself.
-func (e *Entry) checkAuthors() error {
+func (e *Entry) checkAuthors(authorInFeed bool) error {
 	if e.Authors == nil {
-		if e.Source.Authors == nil {
-			return errors.New("no authors set in entry")
+		if !authorInFeed {
+			if e.Source == nil {
+				return errors.New("no authors set in entry")
+			}
+			if e.Source.Authors == nil {
+				return errors.New("no authors set in entry")
+			}
 		}
 	} else {
 		for i, a := range e.Authors {
@@ -135,7 +140,7 @@ func (e *Entry) Check() error {
 		}
 	}
 
-	if err := e.checkAuthors(); err != nil {
+	if err := e.checkAuthors(true); err != nil {
 		return fmt.Errorf("entry %v: %v", e.ID.URI, err)
 	}
 
@@ -208,7 +213,7 @@ func (e *Entry) Check() error {
 		// is not an XML media type [RFC3023], does not begin with "text/", and
 		// does not end with "/xml" or "+xml".
 		mediaType := e.Content.getType()
-		if !isXMLMediaType(mediaType) && !strings.HasPrefix(mediaType, "text/") {
+		if isValidMediaType(mediaType) && !isXMLMediaType(mediaType) && !strings.HasPrefix(mediaType, "text/") {
 			return fmt.Errorf("no summary element of entry %v but media type not xml", e.ID.URI)
 		}
 	}

feed.go

@@ -128,7 +128,7 @@ func (f *Feed) Check() error {
 	// least one atom:author element.
 	if f.Authors == nil {
 		for _, e := range f.Entries {
-			if err := e.checkAuthors(); err != nil {
+			if err := e.checkAuthors(false); err != nil {
 				return fmt.Errorf("no authors set in feed %v: %v", f.ID.URI, err)
 			}
 		}

link.go

@@ -38,16 +38,22 @@ func (l *Link) Check() error {
 		}
 	}
 
-	if strings.Contains(l.Rel, ":") || !isValidIRI(IRI(l.Rel)) {
-		return fmt.Errorf("rel attribute %v of link %v not correctly formatted", l.Rel, l.Href)
+	if l.Rel != "" {
+		if strings.Contains(l.Rel, ":") && !isValidIRI(IRI(l.Rel)) {
+			return fmt.Errorf("rel attribute %v of link %v not correctly formatted", l.Rel, l.Href)
+		}
 	}
 
-	if !isValidMediaType(string(l.Type)) {
-		return fmt.Errorf("type attribute %v of link %v invalid media type", l.Type, l.Href)
+	if l.Type != "" {
+		if !isValidMediaType(string(l.Type)) {
+			return fmt.Errorf("type attribute %v of link %v invalid media type", l.Type, l.Href)
+		}
 	}
 
-	if !isValidLanguageTag(l.HrefLang) {
-		return fmt.Errorf("hreflang attribute %v of link %v invalid language tag", l.Type, l.HrefLang)
+	if l.HrefLang != "" {
+		if !isValidLanguageTag(l.HrefLang) {
+			return fmt.Errorf("hreflang attribute %v of link %v invalid language tag", l.Type, l.HrefLang)
+		}
 	}
 
 	if l.Content == nil {