Decoding of entities with numbers

This ensures that entities such as "½" are decoded properly.
Previously this would be ignored as the regular expression used for this
only matched [a-zA-Z].

This was adapted from PR #111.

lib/oga/xml/entities.rb

@@ -42,7 +42,7 @@ module Oga
       #
       # @return [Regexp]
       #
-      REGULAR_ENTITY = /&[a-zA-Z]+;/
+      REGULAR_ENTITY = /&[a-zA-Z0-9]+;/
 
       ##
       # Regexp for matching XML/HTML entities such as "&".

spec/oga/html/entities_spec.rb

@@ -11,5 +11,9 @@ describe Oga::HTML::Entities do
     it 'decodes λ into λ' do
       described_class.decode('λ').should == 'λ'
     end
+
+    it 'decodes ½ into ½' do
+      described_class.decode('½').should == '½'
+    end
   end
 end