Fix crash with nil XPath variables (#13)

Patch by Alyssa Ross. Thanks!!!

lib/rexml/functions.rb

@@ -135,8 +135,7 @@ def Functions::get_namespace( node_set = nil )
     #
     # An object of a type other than the four basic types is converted to a
     # string in a way that is dependent on that type.
-    def Functions::string( object=nil )
-      object = @@context[:node] if object.nil?
+    def Functions::string( object=@@context[:node] )
       if object.respond_to?(:node_type)
         case object.node_type
         when :attribute
@@ -165,8 +164,6 @@ def Functions::string( object=nil )
               object.to_s
             end
           end
-        when nil
-          ""
         else
           object.to_s
         end

test/rexml/test_functions.rb

@@ -6,6 +6,12 @@
 module REXMLTests
   class FunctionsTester < Test::Unit::TestCase
     include REXML
+
+    def setup
+      super
+      REXML::Functions.context = nil
+    end
+
     def test_functions
       # trivial text() test
       # confuse-a-function
@@ -222,6 +228,19 @@ def test_normalize_space
       assert_equal( [REXML::Comment.new("COMMENT A")], m )
     end
 
+    def test_string_nil_without_context
+      doc = REXML::Document.new(<<-XML)
+      <?xml version="1.0" encoding="UTF-8"?>
+      <root>
+      <foo bar="baz"/>
+      <foo bar=""/>
+      </root>
+      XML
+
+      m = REXML::XPath.match(doc, "//foo[@bar=$n]", nil, { "n" => nil })
+      assert_equal( 1, m.size )
+    end
+
     def test_unregistered_method
       doc = Document.new("<root/>")
       assert_nil(XPath::first(doc.root, "to_s()"))