make sure HEAD requests do not return a body

[lordrhodos]

Suggests a fix for issue #2386

@akrabat @l0gicgate I need some feedback if this is the right approach

[coveralls]

Coverage increased (+0.006%) to 97.825% when pulling 25470cb on lordrhodos:2386-fix-for-head-requests into 715d130 on slimphp:3.x.

[l0gicgate]

A couple of things:

1. You will need to clear out the existing Response body in App::finalize();
2. Add the test case from the 4.x branch as seen here

Thank you!

[l0gicgate]

Everything looks good to me @lordrhodos. Just need @akrabat to have a glance and we can merge and close #2386.

[akrabat]

Im a little twitchy about using the container's Request object, but it seems the best choice here

[akrabat]

Do we need this for Slim 4 too?

[l0gicgate]

@akrabat I already did this for Slim 4 see #2547