Fix test errors

Author: miachm

resources/floatfontsize.ods

@@ -549,7 +549,7 @@ private void readCellText(XmlReaderInstance cellReader, Range range) {
         }
 
         // Empty cells are supposed to be represented by null, so return that if we got no content.
-        if (s.length() > 0 && range.getValue() == null) {
+        if (s.length() > 0 && (range.getValue() == null || range.getValue() instanceof String)) {
             range.setValue(s.toString());
         }
     }

src/com/github/miachm/sods/OdsReader.java

@@ -170,7 +170,7 @@ public Object read(XmlReaderInstance reader) {
         public void write(Object value, XMLStreamWriter writer) throws XMLStreamException {
             if (value instanceof String) {
                 writer.writeAttribute("office:value-type", this.getId());
-                writer.writeAttribute("office:string-value", (String)value);
+                writer.writeAttribute("office:string-value", value.toString());
             }
         }
     },