BUG #15836: Casting 'of' to boolean type should throw an invalid input syntax

The following bug has been logged on the website:

Bug reference:      15836
Logged by:          Yuming Wang
Email address:      wgyumg@gmail.com
PostgreSQL version: 12beta1
Operating system:   linux
Description:

Converting 'of' to a boolean type should throw an invalid input syntax.
Because we said in the documentation that 'of' is not accepted as an input
to the boolean data type:
https://www.postgresql.org/docs/12/datatype-boolean.html
```
postgres=# select cast('of' as boolean);
 bool 
------
 f
(1 row)
```