[5.2] Fix replacing route default parameters.

[butschster]

Route::get('foo/{bar?}')->defaults('bar', 'default');

If we create request http:\\localhost\foo\bar

Before

dd(Request::route()->parameters())

// return 
[
    'bar' => 'default'
]

After

dd(Request::route()->parameters())

// return 
[
    'bar' => 'bar'
]