BUG #13420: Array function contains with null

The following bug has been logged on the website:

Bug reference:      13420
Logged by:          Krzysztof Czajka
Email address:      krzysztof.czajka@turcom.pl
PostgreSQL version: 9.3.8
Operating system:   CentOS release 6.6 (Final)
Description:

Function contains on array with null element isn't work correctly.

SELECT ARRAY[1,4,3]  @> ARRAY[3,1]
true

SELECT ARRAY[ NULL ] @> ARRAY[ NULL ]
false

SELECT ARRAY[ NULL::varchar,'2' ] @> ARRAY[ NULL::varchar ]
false