BUG #16025: pg_default tablespace is removable but not creatable

The following bug has been logged on the website:

Bug reference:      16025
Logged by:          Matthew Kingsbury
Email address:      kingsbury.matthew@gmail.com
PostgreSQL version: 11.5
Operating system:   Ubuntu 18.04 LTS
Description:

I was able to rename pg_default to something else, accidentally. However,
there appears to be no mechanism to replace pg_default nor can one replace
the pg_default after performing this action. E.g. if one runs the command:

ALTER TABLESPACE pg_default
  RENAME TO not_pg_default;

ALTER TABLESPACE not_pg_default
  RENAME TO pg_default;

One encounters this error:
ERROR: unacceptable tablespace name "pg_default"
DETAIL: The prefix "pg_" is reserved for system tablespaces.

Leaving one without the pg_default tablespace.