The compression parameter to PQsslAttribute has never returned the
compression method used, it has always returned "on" or "off since
it was added in commit 91fa7b47. Backpatch through v10.
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/B9EC60EC-F665-47E8-A221-398C76E382C9@yesql.se
Backpatch-through: v10