Make route group namespaces absolute if they begin with a '\'

[Jack97]

Currently, you can register routes to point to a fully qualified class name if it starts with a '\' - which will ignore the namespace that would otherwise be inherited from the route group stack:

Route::get('users', '\Foo\Bar\UsersController@index')

This PR adds the same functionality for route groups:

Route::namespace('\Foo\Bar')->group(function () {
    Route::get('users', 'UsersController@index'); // Foo\Bar\UsersController@index
});

[ntzm]

This will break existing code if a dev has leading backslashes. Should target master instead of 5.6

[taylorotwell]

Indeed a breaking change.