Let local readers and Object methods through the off-quota guard

[Thayorns]

The off-quota guard of Fbe.octo ran for every method called on the client, even those that make no request. So after a commits call took the quota under the threshold, last_response of that very call raised Fbe::OffQuota, and so did auto_paginate, per_page, nil?, inspect and class, as #1226 shows. With the quota unknown, each such call could also cost a /rate_limit request.

Now the guard lets through last_response, auto_paginate and per_page, and any method that every Object has. Methods that call GitHub are still refused off quota.

The new tests drain the quota with one commits call and then read last_response, the paging settings and a random Object method, check that another commits is still refused, and check that per_page costs no /rate_limit request. The tests pass an empty github_token explicitly, so a GITHUB_TOKEN in the environment doesn't change what they see.

Closes #1226

[Thayorns]

@yegor256 take a look please, happy to clarify anything about this change.