IMO, easier to move the logic up the stack into the header dictionary. Requests uses a [case-insensitive dictionary](https://github.com/rmccue/Requests/blob/master/library/Requests/Response/Headers.php) which implements `ArrayAccess`, giving:

    // X-Multiple: first-value
    // x-MULTIPLE: second-value
    $value = $headers['x-multiple'];
    // 'first-value,second-value'
    $values = $headers->getValues('x-multiple');
    // array( 'first-value', 'second-value' )

Originally, it didn't have a distinction here and always used the first form, however thanks to the wonderful world of HTTP specifications, that doesn't always work. Notably, cookie headers will cause problems if you always concatenate with a comma, as commas are used in the old specification in the Expires part.