Cleaned up XPath specs using a shared example.

spec/support/shared_examples.rb

@@ -0,0 +1,9 @@
+shared_examples :node_set do |options|
+  example 'return a NodeSet instance' do
+    @set.is_a?(Oga::XML::NodeSet).should == true
+  end
+
+  example 'return the right amount of rows' do
+    @set.length.should == options[:length]
+  end
+end