Queries with `count(case when [cond] then 1 else null end)` fail to plan

[Micah Wylde]

Queries of the structure

select 
  count(case when action = 'click' then 1 else null end),
from stream
group by tumble(interval '1 second');

Currently fail to plan, with an error like

expected aggregate function, not COUNT(CASE WHEN stream.action = Utf8("click") 
THEN Int64(1) ELSE Int64(NULL) END) AS COUNT(CASE WHEN stream.action = Utf8("click") 
THEN Int64(1) ELSE NULL END)

However, other case statements do work correctly, for example:

sum(case when action = 'click' then 1 else 0 end)

This appears to be an issue with how we identify aggregate expressions within an alias (caused here by the implicit type conversion due to the null)