Added Oga::EntityDecoder

This module removes some of the code duplication needed to determine
what entity decoder to use.

spec/oga/entity_decoder_spec.rb

@@ -0,0 +1,31 @@
+require 'spec_helper'
+
+describe Oga::EntityDecoder do
+  describe 'try_decode' do
+    it 'returns nil if the input is also nil' do
+      described_class.try_decode(nil).should be_nil
+    end
+
+    it 'decodes XML entities' do
+      described_class.try_decode('<')
+        .should == Oga::XML::Entities::DECODE_MAPPING['<']
+    end
+
+    it 'decodes HTML entities' do
+      described_class.try_decode('©', true)
+        .should == Oga::HTML::Entities::DECODE_MAPPING['©']
+    end
+  end
+
+  describe 'decode' do
+    it 'decodes XML entities' do
+      described_class.decode('<')
+        .should == Oga::XML::Entities::DECODE_MAPPING['<']
+    end
+
+    it 'decodes HTML entities' do
+      described_class.decode('©', true)
+        .should == Oga::HTML::Entities::DECODE_MAPPING['©']
+    end
+  end
+end