CASE folding can produce wrong expression

This is a case that was not covered in the testsuite. For the following query:

select (CASE WHEN key = value THEN '1' WHEN true THEN '0' ELSE NULL END)
from src

Currently, we end up folding the select expression to '0', as we fail bail out in the second statement and fail to account that there are two different possible values for the CASE expression ('1' and '0').