Do not pass $next handler to final request handler

[clue]

The last handler in a "handler array" is in fact a "final handler" and by definition not a "middleware handler". As such, all but the last entry should receive a $next handler. This is in line with how a "normal request handler" also doesn't receive a $next handler.

Builds on top of the work done by @jsor in #300.
Supersedes / closes #300