Align implementation with Rack SPEC

[ioquatix]

And update specs/expand testing.

[grape-bot]

	1 Warning
⚠️	Unless you’re refactoring existing code, please update CHANGELOG.md.

Here's an example of a CHANGELOG.md entry:

* [#1956](https://github.com/ruby-grape/grape/pull/1956): Align implementation with rack spec - [@ioquatix](https://github.com/ioquatix).

Generated by 🚫 danger

[ioquatix]

I think this is okay to merge. @dblock

[dblock]

👍 merged, although I do worry about .to_a causing unexpected regressions (eg. streaming response)

[ioquatix]

Rack middleware must return an array of [status, headers, body] and all the .to_a method is doing is forcing that. It doesn't change any of the individual fields, e.g. body is still body. You shouldn't be returning a rich object in order to support streaming.

That being said, I understand your concern.

[nbulaj]

Actually this changes broked our app after upgrade, check this one: #2030 (comment)
Could you check please @ioquatix ?

[ghiculescu]

Should we add something about this to https://github.com/ruby-grape/grape/blob/master/CHANGELOG.md#130-20200111

I ended up here looking into #1966 (while upgrading Rack due to its recent CVE) and it'd be great if the changelog called that out somehow

[dnesteryuk]

@ghiculescu of course, go ahead! 🙂

[ghiculescu]

@ghiculescu of course, go ahead! 🙂

#2059