[5.6] Fix whereTime method for PostgresGrammar with correct casting.

[pinguinjkeke]

Similar as whereDate, whereTime should cast timestamp to time like created_at::time.

Currently ->whereTime() throws an SQL grammar error, so it doesn't break anything:

ERROR: syntax error at or near "time"

It's a bugfix. So I think it should be merged to current LTS 5.5 branch too, I think.

Link: https://www.postgresql.org/docs/9.6/static/functions-datetime.html

[sisve]

Could we get the PR for 5.5 reopened and merged too? #23318

[pinguinjkeke]

@sisve yeah, it would be cool