[8.x] Fix assertDownload if testing against a filename with spaces in it

[fabio-ivona]

This PR will fix an unexpected assertion failures when trying to check a TestResponse download filename:

let's suppose we are testing a method that returns a storage download:

Storage::download('file.txt', 'test file.txt');

the generated StreamedResponse will have this header:

content-disposition: attachment; filename="test file.txt"

as TestResponse checks ->assertDownload('test file.txt') against that header, it will trow a failure because

test file.txt != "test file.txt"

the only way I found to make this to pass is to check ->assertDownload('"test file.txt"')

My PR aims to fix this by adding a quotes trimming to the assertion