Added ips benchmark for NodeSet#push

benchmark/xml/node_set/push_bench.rb

@@ -0,0 +1,17 @@
+require_relative '../../benchmark_helper'
+
+# Assigning to an Array first saves the need (and overhead) of calling #push for
+# every iteration.
+initial = 20_000.times.map do |number|
+  Oga::XML::Element.new(:name => number.to_s)
+end
+
+initial = Oga::XML::NodeSet.new(initial)
+
+Benchmark.ips do |bench|
+  bench.report 'XML::NodeSet#push' do
+    new_set = Oga::XML::NodeSet.new
+
+    initial.each { |node| new_set << node }
+  end
+end