[Routing] Remove usage of deprecated _scheme requirement

This is exact the same commit as in #9585, which was not merged due to my fault. Sorry for the noise.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8898, #8176
| License       | MIT
| Doc PR        | 

I removed all usages of the deprecated _scheme requirement inside the Routing Component.
Most parts were pretty easy and after multiple refactorings I came up with the solution to have a Route::hasScheme() method and check against this method.

I also checked for performance and after trying in_array, arra_flip+isset and foreach, the last one was clearly the winner.
https://gist.github.com/Danez/7609898#file-test_performance-php

I also adjusted all tests that test '_scheme' to also check the new schemes-requirement.