non-buffered responses fixes

[1ma]

• Fix potential casting from array to string in Response::withHeader()
• Fix non buffered responses when calling Response::withAddedHeader() and Response::withoutHeader()

Reference: slimphp/Slim-Psr7#39 (comment)

[coveralls]

Coverage increased (+0.3%) to 97.606% when pulling 6914d32 on 1ma:fix-nonbuffer-headers into e6f75e8 on slimphp:3.x.

[l0gicgate]

Can you PR this on Slim PSR-7 as well please? Also we should add tests cases for the newly introduced code.

[1ma]

The merged PR against slim/psr7 that I referenced in the first comment already has these fixes. Also, regarding the difficulty of testing this feature: #2504 (comment)

Unfortunately neither of the 2 original PRs had any (real) tests for this.

[1ma]

That'd be it. I've done some cleanup in the testing autoloading, too. These overrides should only be loaded from tests/bootstrap.php, not at each composer install. Now to port these new tests back to Slim-Psr7...

[l0gicgate]

I'm somewhat worried we might break some builds in the wild as seen in #2669 because we're moving the assets into bootstrap.php but I guess we'll have to deal with it when the issues come in. Great work @1ma!