[8.x] Fixes middleware() not accepting Stringable

[DarkGhostHunter]

What?

Fixes declaring routes as Route::get()->middleware() with Stringable objects. As seen on TV:

I may need to revert this already @DarkGhostHunter

This simple test doesn't even work locally:

Originally posted by @taylorotwell in #39439 (comment)

This also removes son unnecessary checks for middleware declaration, as is expected to be an array, or a string (which gets converted to a one-member array anyway).

[DarkGhostHunter]

Can someone explain why test fails because of yes?

[driesvints]

@DarkGhostHunter restarted them for ya

[DarkGhostHunter]

Ty. Please don't crash now.

[taylorotwell]

Thanks

[DarkGhostHunter]

👌