Added NodeSet#concat.

This method allows two sets to be concatenated together without the need of
creating a 3rd node set (as is the case with NodeSet#+).

lib/oga/xml/node_set.rb

@@ -183,6 +183,15 @@ module Oga
         return self.class.new(to_a | other.to_a)
       end
 
+      ##
+      # Adds the nodes of the given node set to the current node set.
+      #
+      # @param [Oga::XML::NodeSet] other
+      #
+      def concat(other)
+        other.each { |node| push(node) }
+      end
+
       ##
       # Removes the current nodes from their owning set. The nodes are *not*
       # removed from the current set.

spec/oga/xml/node_set_spec.rb

@@ -235,6 +235,22 @@ describe Oga::XML::NodeSet do
     end
   end
 
+  context '#concat' do
+    before do
+      n1 = Oga::XML::Element.new(:name => 'a')
+      n2 = Oga::XML::Element.new(:name => 'b')
+
+      @set1 = described_class.new([n1])
+      @set2 = described_class.new([n2])
+    end
+
+    example 'concatenate two node sets' do
+      @set1.concat(@set2)
+
+      @set1.length.should == 2
+    end
+  end
+
   context '#remove' do
     before do
       owner = Oga::XML::Element.new