Bugs in Feather, the Redeemed

[padfoothelix]

While working on implementing Gandalf of the Secret Fire, I've found a few problems with Feather, the Redeemed.
I've made a pull request ( #13226) to propose a fix. I'll detail the problems here ; please let me know if this isn't the right way to do this.

Some of these problems also affect Lilah, Undefeated Slingshot, which was implemented based on Feather.

Problems :

1. Feather's replacement effect never checks if the spell changing zone is the same that triggered its ability. This leads to the following bug
   a. cast an instant or sorcery on any creature you control with Feather on the battlefield
   b. let the ability resolve
   c. before the instant/sorcery resolves, any new instant or sorcery you cast will trigger the replacement effect, be exiled and sent back to your hand at eot, which it clearly shouldn't.

2. Feather only checks that the spell is an instant or sorcery in the replacement effect, not in the triggered ability. This means that casting say Rancor one of your creatures with Feather present will trigger Feather. The replacement effect will not apply, but the ability should nonetheless not trigger.

3. Feather's effect seems to last forever, even after it has exiled and bounced the original spell. It never applies again, but that means a whole lot of tests are happening needlessly.

I have tried to fixed these issues in the pull request referenced above ; please advise if the fix isn't correct.