Fix #28 by using TopScope instead of null

Author: ashawley

src/main/scala/scala/xml/PrettyPrinter.scala

@@ -244,8 +244,8 @@ class PrettyPrinter(width: Int, step: Int) {
    *  @param pscope the namespace to prefix mapping
    *  @return      the formatted string
    */
-  def format(n: Node, pscope: NamespaceBinding = null): String =
-    sbToString(format(n, pscope, _))
+  def format(n: Node, pscope: NamespaceBinding = TopScope): String = {
+    sbToString(format(n, pscope, _)) }
 
   /**
    * Returns a formatted string containing well-formed XML.