minor problem in boolean cast

Hi

I noticed that when casting a string to boolean value with input 'of' it still cast it to 'f'. I think with 'of', it should give an error because 'off' is the expected candidate. This may not be intended so I made a simple patch to address this. 

```

postgres=# select cast('of' as boolean);

bool

------

f

(1 row)

```