Fixed index check in Node#next

An index can/should never be equal the length of a NodeSet, thus we
should use "<" here instead of "<=".

lib/oga/xml/node.rb

@@ -70,7 +70,7 @@ module Oga
         index  = node_set.index(self) + 1
         length = node_set.length
 
-        index <= length ? node_set[index] : nil
+        index < length ? node_set[index] : nil
       end
 
       # Returns the previous element node or nil if there is none.

spec/oga/xml/node_spec.rb

@@ -86,7 +86,7 @@ describe Oga::XML::Node do
       @n1.next.should == @n2
     end
 
-    it 'returns nil if there is no previous node' do
+    it 'returns nil if there is no next node' do
       @n2.next.nil?.should == true
     end
   end