Wrap nested Nodes::Grouping in brackets only once

Tested on Rails 4.1 application with mysql2.  

```ruby
# Before:
Developer.where(id: [1,nil])  
# SELECT `developers`.* FROM `developers`  WHERE ((`developers`.`id` = 1 OR `developers`.`id` IS NULL))  

# After:  
Developer.where(id: [1,nil])  
# SELECT `developers`.* FROM `developers`  WHERE (`developers`.`id` = 1 OR `developers`.`id` IS NULL)

  
# Unfortunately, there are still extra brackets in the following queries, but I'm tired debugging Arel
# Notice 2 spaces before WHERE as well
Developer.where.not(id: [1,2])  
# SELECT `developers`.* FROM `developers`  WHERE (`developers`.`id` NOT IN (1, 2))
Developer.where.not(id: [1,nil]) 
#  SELECT `developers`.* FROM `developers`  WHERE (NOT ((`developers`.`id` = 1 OR `developers`.`id` IS NULL)))