Raise for non node sets in the sum() function.

According to the XPath spec this function *can only* take node sets, nothing
else. Lets actually enforce that.

lib/oga/xpath/evaluator.rb

@@ -1174,6 +1174,10 @@ module Oga
         nodes = process(expression, context)
         sum   = 0.0
 
+        unless nodes.is_a?(XML::NodeSet)
+          raise TypeError, 'sum() can only operate on NodeSet instances'
+        end
+
         nodes.each do |node|
           sum += node.text.to_f
         end

spec/oga/xpath/evaluator/calls/sum_spec.rb

@@ -19,5 +19,11 @@ describe Oga::XPath::Evaluator do
     example 'return zero by default' do
       @evaluator.evaluate('sum(foo)').should be_zero
     end
+
+    example 'raise a TypeError for non node set arguments' do
+      block = lambda { @evaluator.evaluate('sum("foo")') }
+
+      block.should raise_error(TypeError)
+    end
   end
 end