Fixed XML entity encoding/decoding ordering.

Thanks to @krasnoukhov for providing the initial patch, which this commit is
largely based on.

This fixes #49.

lib/oga/xml/entities.rb

@@ -4,15 +4,15 @@ module Oga
       ##
       # Hash containing XML entities and the corresponding characters.
       #
-      # The `&` mapping must come first to ensure proper conversion of non
+      # The `&` mapping must come last to ensure proper conversion of non
       # encoded to encoded forms (see {Oga::XML::Text#to_xml}).
       #
       # @return [Hash]
       #
       DECODE_MAPPING = {
-        '&' => '&',
         '&lt;'  => '<',
-        '&gt;'  => '>'
+        '&gt;'  => '>',
+        '&amp;' => '&'
       }
 
       ##
@@ -20,7 +20,11 @@ module Oga
       #
       # @return [Hash]
       #
-      ENCODE_MAPPING = DECODE_MAPPING.invert
+      ENCODE_MAPPING = {
+        '&' => '&amp;',
+        '>' => '&gt;',
+        '<' => '&lt;'
+      }
 
       ##
       # Decodes XML entities.

spec/oga/xml/entities_spec.rb

@@ -13,6 +13,22 @@ describe Oga::XML::Entities do
     example 'decode > into >' do
       described_class.decode('>').should == '>'
     end
+
+    example 'decode > into >' do
+      described_class.decode('>').should == '>'
+    end
+
+    example 'decode &> into &>' do
+      described_class.decode('&>').should == '&>'
+    end
+
+    example 'decode &amp;lt; into <' do
+      described_class.decode('&amp;lt;').should == '&lt;'
+    end
+
+    example 'decode &amp;&amp;lt; into &<' do
+      described_class.decode('&amp;&amp;lt;').should == '&&lt;'
+    end
   end
 
   context 'encode' do
@@ -27,5 +43,13 @@ describe Oga::XML::Entities do
     example 'encode > as &gt;' do
       described_class.encode('>').should == '&gt;'
     end
+
+    example 'encode &gt; as &amp;gt;' do
+      described_class.encode('&gt;').should == '&amp;gt;'
+    end
+
+    example 'encode &lt; as &amp;lt;' do
+      described_class.encode('&lt;').should == '&amp;lt;'
+    end
   end
 end