Default to TopScope if printing with StringBuilder

Author: chrisloy

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

@@ -203,7 +203,7 @@ class PrettyPrinter(width: Int, step: Int) {
    * @param sb   the stringbuffer to append to
    */
   def format(n: Node, sb: StringBuilder) { // entry point
-    format(n, null, sb)
+    format(n, TopScope, sb)
   }
 
   def format(n: Node, pscope: NamespaceBinding, sb: StringBuilder) { // entry point

src/test/scala/scala/xml/XMLTest.scala

@@ -846,4 +846,15 @@ expected closing tag of foo
     assertEquals(expected, pp.formatNodes(x))
   }
 
+  @UnitTest
+  def nodeStringBuilder: Unit = {
+    val x = {
+        <x:foo xmlns:x="abc"/>
+    }
+    val pp = new PrettyPrinter(80, 2)
+    val expected = """<x:foo xmlns:x="abc"/>"""
+    val sb = new StringBuilder
+    pp.format(x, sb)
+    assertEquals(expected, sb.toString)
+  }
 }