Soften test for incomplete edit POST

Do not reject POSTs from forms that were rendered before
wpUltimateParam was added.

There is also an open question about whether we should respect
kludgey scripts which have hardcoded form params.

See Umherirrender's comment on
change I78f1bbcc9bb092568fbfbec19c08d697c9ae75f8

Change-Id: I28bd6d45442bc3ba352f0de7a50709adb98b1c50
(cherry picked from commit 05a19c5ba7145bed7c2a323e5f6c9e9babea53a0)