Responding to all verbs in a namespace causes Options to start failing

[myxoh]

If a namespace responds to: POST, PUT, PATCH, GET, DELETE on a base resource - the Options preflight gives back a 404. If you remove any of the verbs it works as expected.

Spec reproducing the behaviour:
#2176

I'll investigate what's going on and attempt to fix

[myxoh]

OK after some investigation - I've found the root cause being this line:
return if not_allowed_methods.empty?
(Now on: lib/grape/api/instance.rb after my changes but originally introduced:)
https://github.com/ruby-grape/grape/commit/e54b8b0e40d34b64df65c2002f0dcc8be7edfcf3

[myxoh]

Coming from this PR: #1263

[myxoh]

OK managed to find a way of squaring the problem originally being solved on that PR and my problem.. Adding the fix soon