XmlMini.rename_key emits valid xml with dasherize

This resolves issues for libraries which use '_' prefixed keys in their
attributes hash, such as Mongoid. A key like "_id" or "_type" will no
longer be converted to "<-id>" and "<-type>".

Signed-off-by: wycats <wycats@gmail.com>

activesupport/lib/active_support/xml_mini.rb

@@ -129,12 +129,16 @@ module ActiveSupport
       camelize  = options.has_key?(:camelize) && options[:camelize]
       dasherize = !options.has_key?(:dasherize) || options[:dasherize]
       key = key.camelize  if camelize
-      key = key.dasherize if dasherize
+      key = _dasherize(key) if dasherize
       key
     end
 
     protected
 
+    def _dasherize(key)
+      key.gsub(/(?!^[_]*)_(?![_]*$)/, '-')
+    end
+
 	  # TODO: Add support for other encodings
     def _parse_binary(bin, entity) #:nodoc:
       case entity['encoding']

activesupport/test/test_xml_mini.rb

@@ -0,0 +1,49 @@
+require 'abstract_unit'
+require 'active_support/xml_mini'
+
+class XmlMiniTest < Test::Unit::TestCase
+  def test_rename_key_dasherizes_by_default
+    assert_equal "my-key", ActiveSupport::XmlMini.rename_key("my_key")
+  end
+
+  def test_rename_key_does_nothing_with_dasherize_true
+    assert_equal "my-key", ActiveSupport::XmlMini.rename_key("my_key", :dasherize => true)
+  end
+
+  def test_rename_key_does_nothing_with_dasherize_false
+    assert_equal "my_key", ActiveSupport::XmlMini.rename_key("my_key", :dasherize => false)
+  end
+
+  def test_rename_key_camelizes_with_camelize_true
+    assert_equal "MyKey", ActiveSupport::XmlMini.rename_key("my_key", :camelize => true)
+  end
+
+  def test_rename_key_camelizes_with_camelize_true
+    assert_equal "MyKey", ActiveSupport::XmlMini.rename_key("my_key", :camelize => true)
+  end
+
+  def test_rename_key_does_not_dasherize_leading_underscores
+    assert_equal "_id", ActiveSupport::XmlMini.rename_key("_id")
+  end
+
+  def test_rename_key_with_leading_underscore_dasherizes_interior_underscores
+    assert_equal "_my-key", ActiveSupport::XmlMini.rename_key("_my_key")
+  end
+
+  def test_rename_key_does_not_dasherize_trailing_underscores
+    assert_equal "id_", ActiveSupport::XmlMini.rename_key("id_")
+  end
+
+  def test_rename_key_with_trailing_underscore_dasherizes_interior_underscores
+    assert_equal "my-key_", ActiveSupport::XmlMini.rename_key("my_key_")
+  end
+
+  def test_rename_key_does_not_dasherize_multiple_leading_underscores
+    assert_equal "__id", ActiveSupport::XmlMini.rename_key("__id")
+  end
+
+  def test_rename_key_does_not_dasherize_multiple_leading_underscores
+    assert_equal "id__", ActiveSupport::XmlMini.rename_key("id__")
+  end
+
+end