Make "not" apply to the whole sub-expression when generating sql.

lib/arel/visitors/to_sql.rb

@@ -192,7 +192,7 @@ def visit_Arel_Nodes_On o
       end
 
       def visit_Arel_Nodes_Not o
-        "NOT #{visit o.expr}"
+        "NOT (#{visit o.expr})"
       end
 
       def visit_Arel_Table o

test/visitors/test_to_sql.rb

@@ -40,7 +40,13 @@ module Visitors
 
       it "should visit_Not" do
         sql = @visitor.accept Nodes::Not.new(Arel.sql("foo"))
-        sql.must_be_like "NOT foo"
+        sql.must_be_like "NOT (foo)"
+      end
+
+      it "should apply Not to the whole expression" do
+        node = Nodes::And.new @attr.eq(10), @attr.eq(11)
+        sql = @visitor.accept Nodes::Not.new(node)
+        sql.must_be_like %{NOT ("users"."id" = 10 AND "users"."id" = 11)}
       end
 
       it "should visit_As" do