Fix the use of Joomla\Http\Response

[SharkyKZ]

Summary of Changes

Fixes the use Joomla\Http\Response where the values of individual headers are stored as arrays since v2 of the joomla/http package. Closes #19.
Updates to use joomla/http v2 syntax only since v1 support was dropped for some reason in 59268f5.
Updates tests to also use Joomla\Http\Response class (the use of stdClass is what allowed the error to slip by).

Testing Instructions

Use the class. Before patch, TypeError is thrown in src\Client.php:116 on PHP 8:

strpos(): Argument #1 ($haystack) must be of type string, array given 

On older PHP versions, a warning is emitted and JSON header detection always fails:

Warning: strpos() expects parameter 1 to be string, array given

Documentation Changes Required

No.