[5.5] Restore Non Static Method Signature to Illuminate\Routing\Router::prepareResponse

[astorm]

(This is my first pull request to `laravel/framework, if I'm doing something wrong my apologies --
it's non-intentional!)

This pull request restores the non-static method signature of Illuminate\Routing\Router::prepareResponseObject from previous Laravel versions, adds a new static method named Illuminate\Routing\Router::prepareResponseObject, and then replaces the one explicit static call to prepareResponse in the exception handler.

Reason for this Request: I'm working with/at a company that has a PHP Extension (C code compiled into a .so file and loaded via a php.ini file). The change in this method signature caused a host of problems that makes the .so extension unusable in Laravel 5.5 (happy to discuss these offline). We're working on a fix on our end, but it would be fantastic for our Laravel using customers if we could get the old method signature back.

Also -- acknowledging that the time to bring this up was very likely before Laravel 5.5's release. I'm coming into this process late, but we're working hard on our regression tests so this doesn't happy again :) Thanks for the time/attention!