Fix graphml parsing (NumberFormatException)

Fix NumberFormatException when reading characters. Pass getData() instead of the Characters event.
See graphstream#363

src/org/graphstream/stream/file/FileSourceXML.java

@@ -460,7 +460,7 @@ protected String __characters() throws IOException, XMLStreamException {
 			e = getNextEvent();
 
 			while (e.getEventType() == XMLEvent.CHARACTERS) {
-				buffer.append(e.asCharacters());
+				buffer.append(e.asCharacters().getData());
 				e = getNextEvent();
 			}