Test with rack-test 2.0

[terceiro]

Those two tests endpoint_spec are in the end testing internal details of rack-test. Change them so that they still test what was intended, but in a more robust way that does not break on changes on the implementation details of rack-test.

Fixes: https://github.com/ruby-grape/grape/issues/2278

[grape-bot]

	1 Warning
:warning:	Unless you're refactoring existing code or improving documentation, please update CHANGELOG.md.

	1 Message
:book:	We really appreciate pull requests that demonstrate issues, even without a fix. That said, the next step is to try and fix the failing tests!

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

* [#2279](https://github.com/ruby-grape/grape/pull/2279): Test with rack-test 2.0 - [@terceiro](https://github.com/terceiro).

Generated by :no_entry_sign: Danger

[dblock]

Want to finish this @terceiro ?

[terceiro]

was this closed by mistake? I think we should get support for rack-test 2.0 at some point

[terceiro]

no, sorry for the noise. that MR does seem to address the issues here