Fix #not to stop wrapping in a grouping node

lib/arel/nodes/node.rb

@@ -10,7 +10,7 @@ module Arel
       # Factory method to create a Nodes::Not node that has the recipient of
       # the caller as a child.
       def not
-        Nodes::Not.new Nodes::Grouping.new self
+        Nodes::Not.new self
       end
 
       ###

test/nodes/test_not.rb

@@ -6,13 +6,10 @@ module Arel
       describe '#not' do
         it 'makes a NOT node' do
           attr = Table.new(:users)[:id]
-          left  = attr.eq(10)
-          right = attr.eq(11)
-          node  = left.or right
-          node.expr.left.must_equal left
-          node.expr.right.must_equal right
-
-          node.or(right).not
+          expr  = attr.eq(10)
+          node  = expr.not
+          node.must_be_kind_of Not
+          node.expr.must_equal expr
         end
       end
     end