Don't use Array#uniq in NodeSet#initialize.

Removing this makes the process of parsing larger XML documents a bit faster.
The downside is that NodeSet#initialize will no longer filter out duplicate
nodes, though this is not something Oga itself relies upon.

Methods such as NodeSet#push still do ignore elements already present.

lib/oga/xml/node_set.rb

@@ -44,7 +44,7 @@ module Oga
       # @param [Oga::XML::NodeSet] owner The owner of the set.
       #
       def initialize(nodes = [], owner = nil)
-        @nodes = nodes.uniq
+        @nodes = nodes
         @owner = owner
 
         @nodes.each { |node| take_ownership(node) } if owner