Fixes == for comparing values of 2 dynamic nodes.

### Example:
```ruby
class Foo
  include OM::XML::Document

  set_terminology do |t|
      t.root(:path => 'foo')
      t.bar
      t.bar2
  end

  def self.xml_template
      Nokogiri::XML.parse("<foo/>")
  end
end

f = Foo.new
f.foo.bar = "a"
f.foo.bar2 = "a"

# before
f.foo.bar == f.foo.bar2
# => false

# after
f.foo.bar == f.foo.bar2
# => true
```