change / operator behaviour

this changes hstore division 

```SQL
SELECT 'a=>3, b=>2'::inthstore / 'a=>2, b=>5'::inthstore;
                         ?column?                         
----------------------------------------------------------
 "a"=>"1.5000000000000000", "b"=>"0.40000000000000000000"
(1 row)

SELECT 'a=>3, b=>2'::inthstore / 'a=>2, b=>2, c=>5'::inthstore;
                              ?column?                               
---------------------------------------------------------------------
 "a"=>"1.5000000000000000", "b"=>"1.00000000000000000000", "c"=>NULL
(1 row)

SELECT 'a=>3, b=>2, c=>5'::inthstore / 'a=>2, b=>2'::inthstore;
ERROR:  Keys of the numerator doesn't match denominator.
DETAIL:  extra keys are {c}
HINT:  You may want to slice before
```