Handle EOF in limited ask prompts

[JoelTowell]

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Problem

ask_simply already returns nil when input reaches EOF (here). That behaviour was added in #252, after #248 reported that Ctrl-D raised while handling a plain prompt.

ask_filtered uses ask_simply for limited_to prompts, but passes the result straight into answer_match (here). So EOF does not take the same path as an unlimited prompt. The case-sensitive path keeps reasking because nil does not match one of the allowed answers. The case-insensitive path raises because answer_match calls downcase on the answer (here).

Closes #935.

Reproduction

The first commit on this branch adds failing examples for this. It can be checked out independently to see the failure.

Solution

Return from ask_filtered when ask_simply returns nil, matching the EOF behaviour already used by plain prompts.