Unexpected behavior from using default config value

I encountered this problem while I was trying to enable SSL on my
postgresql server. Since I was satisfied with the default values for the
"ssl_key_file" and "ssl_cert_file" settings I chose to not configure them
-- I simply turned on "ssl" and copied over the files to the default
locations. However, I kept getting certificate errors on the client.
Examining the certificate sent by the server using `openssl s_client
-starttls postgres -connect "$HOSTNAME:5432"` revealed that the server was
sending some auto-generated cert instead of the one in "server.crt".
Setting the "ssl_key_file" and "ssl_cert_file" settings explicitly to their
default value fixed the problem. From the documentation I would expect that
missing values for config keys would fallback to their default value;
instead it seems that postgresql has some undocumented fallback behavior in
this case.