Added spec for not() in a predicate

spec/oga/xpath/compiler/calls/not_spec.rb

@@ -4,8 +4,11 @@ describe Oga::XPath::Compiler do
   describe 'not() function' do
     before do
       @document = parse('<root>foo</root>')
+
+      @root = @document.children[0]
     end
 
+    describe 'at the top-level' do
       it 'returns false when the argument is a non-zero integer' do
         evaluate_xpath(@document, 'not(10)').should == false
       end
@@ -22,4 +25,11 @@ describe Oga::XPath::Compiler do
         evaluate_xpath(@document, 'not(foo)').should == true
       end
     end
+
+    describe 'in a predicate' do
+      it 'returns a NodeSet containing all matching nodes ' do
+        evaluate_xpath(@document, 'root[not(node())]').should == node_set(@root)
+      end
+    end
+  end
 end