In commit 87259588 I (Álvaro) tried to rationalize the determination
of tablespace to use for partitioned tables, but failed to handle the
default_tablespace case.  Repair and add proper tests.

Author: Amit Langote, Rushabh Lathia
Reported-by: Rushabh Lathia
Reviewed-by: Amit Langote, Álvaro Herrera
Discussion: https://postgr.es/m/CAGPqQf0cYjm1=rjxk_6gU0SjUS70=yFUAdCJLwWzh9bhNJnyVg@mail.gmail.com