[gxml] Fixed GomCollection.validate_append()

[Daniel Espinosa Ortiz <despinosa src gnome org>]

commit 328a8047a1f01889591ae0a5a984a587211aedbb
Author: Daniel Espinosa <esodan gmail com>
Date:   Thu Feb 9 17:47:11 2017 -0600

    Fixed GomCollection.validate_append()
    
    Added some documentation

 gxml/GomCollections.vala |    9 ++++++---
 1 files changed, 6 insertions(+), 3 deletions(-)
---
diff --git a/gxml/GomCollections.vala b/gxml/GomCollections.vala
index c230847..e636295 100644
--- a/gxml/GomCollections.vala
+++ b/gxml/GomCollections.vala
@@ -366,8 +366,11 @@ public class GXml.GomHashMap : GXml.BaseCollection, GXml.GomCollection {
     return _element.child_nodes.get (i) as DomElement;
   }
   /**
-   * Validates if given element has a key property set, if so
-   * adds a new key pointing to given index and returns true.
+   * Validates if given element has a {@link GomHashMap.attribute_key} set,
+   * if so adds a new key pointing to given index and returns true.
+   *
+   * Attribute should be a valid {@link DomElement} attribute or
+   * a {@link GomObject} property identified using a nick with a '::' prefix.
    *
    * Return: false if element should not be added to collection.
    */
@@ -382,7 +385,7 @@ public class GXml.GomHashMap : GXml.BaseCollection, GXml.GomCollection {
     key = (element as DomElement).get_attribute (attribute_key);
     if (key == null)
     key = (element as DomElement).get_attribute (attribute_key.down ());
-    if (key == null) assert_not_reached ();
+    if (key == null) return false;
 #if DEBUG
     message ("Attribute key value: "+key);
 #endif