Default behavior for determining nobs

I'm a huge fan of leveraging `mle2` for fitting complex models. However, I have run into multiple instances where I obtain values for downstream quantities like AICc which don't make sense.

Approximately 99 times out of 100, this is because the value being determined internally for `nobs` is incorrect. Because the object passed to the function using the argument `data` could hypothetically take just about any form, it seems to me that the assumption inherent in the following line of code is only sometimes guaranteed to be correct.

https://github.com/bbolker/bbmle/blob/918d335be65296cd5acbe82b76d694f168ca36a3/R/mle.R#L587

Start at the linked line in R/mle.R around line 587 and trace how mle2 determines nobs from the data argument. Reproduce the incorrect AICc and nobs values with the relevant data forms, then establish and test a default behavior that remains correct across those cases.

Written by the indexing model from the issue text.