Memoize falsy values

[spraints]

Ruby treats nil and false the same when they're on the left hand side of a ||=. There are a couple of slow bits of code that are supposed to run once, but could run on each call, if they return nil or false. This branch changes them to be more explicit about how they handle their instance variables to ensure that the falsy results don't lead to a retry on the next call.

• #dns - This can be an array of results or nil. Before this branch, @dns == nil initially and if the dns results should be nil. This branch changes the latter case to set @dns = false, and it checks explicitly for @dns.nil? to know if it should actually look up any values.
• #served_by_pages? - This will return true or false. Before this branch, @served_by_pages would be nil on init and a boolean after the check had been run. The check ran any time @served_by_pages was nil or false. This branch changes it so that the check only runs when @served_by_pages.nil?.

cc @benbalter

[benbalter]

TIL. Thanks for taking the time to document this as well... learned a lot. 👍

[benbalter]

Hey, also to call myself out, I should have left #29 for at least a day. Thought it was simple enough, and was trying to fix an internal error, but neither are an excuse for not being collaborative. Thanks for the 👀 despite my best efforts. 😸

[spraints]

Hey, also to call myself out, I should have left #29 for at least a day.

Eh, this is no big deal, most of the time. #29 was an improvement, and this PR builds on that to make it a little bit better. Review and iteration can span multiple PRs.