MountainHiker,
That's hard to believe! You're so good at Googling! OK. Here goes:
"Rule": Nominative case
![Idea [I]](/emoticons/emotion-55.gif)
for a predicate nominative.
It is/was/has been ... (I, you, he, she, it, we, you, they).
I gave the conservative (prescriptive) answer in case the question had to do with "English tests". One never knows the real purpose of the question, nor the specific needs of the questioner.
The objective case [me] is accepted by some because people usually say it that way.
It is/was/has been ... (me, you, him, her, it, us, you, them).
Most of the borderline, debatable, or controversial cases are textbook sentences, just to present a complete picture of English grammar, no matter how unusual the combinations and permutations.
In the living language, in my opinion, these sorts of sentences do not often occur. Are they avoided because they are borderline cases? I don't know.
So the "real" English way of expressing the thought in question is
"I'm the one who/that asked to go to the store, not you."
"I / me" problem solved!
Q.E.D.