handle implicit namespaces

Signed-off-by: Florian Thienel <florian@thienel.org>
diff --git a/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/dom/Element.java b/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/dom/Element.java
index e3b0402..65dd01d 100644
--- a/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/dom/Element.java
+++ b/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/dom/Element.java
@@ -290,6 +290,10 @@
 	public String getNamespacePrefix(final String namespaceURI) {
 		if (namespaceURI == null)
 			return null;
+		if (Namespace.XML_NAMESPACE_URI.equals(namespaceURI))
+			return Namespace.XML_NAMESPACE_PREFIX;
+		if (Namespace.XMLNS_NAMESPACE_URI.equals(namespaceURI))
+			return Namespace.XMLNS_NAMESPACE_PREFIX;
 		for (Entry<String, String> entry: namespaceDeclarations.entrySet())
 			if (entry.getValue().equals(namespaceURI))
 				return entry.getKey();