active record conditions to deal with nils in arrays

currently, this:

MyModel.find(:all,:conditions => {:some_param => [nil]})

returns differently to:

MyModel.find(:all,:conditions => {:some_param => nil})

(the difference being whether the nil is in an array or not)

This is due to the situation where this will return the expected result set:

select * from events where venue_id is null

but this will return an empty set:

select * from events where venue_id in (null)

I don't have access to anything other than MySQL databases so can't test whether this affects all database vendors.

The following demonstrates the behaviour:

SomeActiveRecordModel.send(:sanitize_sql_hash_for_conditions,{:some_param => [nil]})

the existing ActiveRecord will return:

`events`.`some_param` IN (NULL)

my patch changes it to return:

(`events`.`some_param` IN (NULL) OR `events`.`some_param` IS NULL)

This is my first patch so please forgive and inform me if something is not as it should be.