In postgres_fdw, allow CASE expressions to be pushed to the remote server.

In postgres_fdw, allow CASE expressions to be pushed to the remote server.

This is simple enough except for the need to check whether CaseTestExpr
nodes have a collation that is not derived from a remote Var.  For that,
examine the CASE's "arg" expression and then pass that info down into the
recursive examination of the WHEN expressions.

Alexander Pyhalov, reviewed by Gilles Darold and myself

Discussion: https://postgr.es/m/fda09032e90d85d9b726a41e03f9097f@postgrespro.ru