Regarding multiple header values for the same field. I would expect to be able to do the following to access an array of un-folded header lines:

```php
<?php
// ... do request stuff ...
$request->addHeader('X-My-Header', 42);
$request->addHeader('X-My-Header', 43);
$headerArray = $request->getHeader('X-My-Header');

assert(is_array($headerArray));
assert(count($headerArray) === 2);
assert($headerArray[0] === 42);
assert($headerArray[1] === 43);
```

Enforcing header folding by only returning an array of strings doesn't seem like a good idea.