PHP 8.1: improve input validation for Requests_Transport_(fsockopen|cURL)

[jrfnl]

PHP 8.1 deprecates passing null to non-nullable parameters for PHP native functions.
Ref: https://wiki.php.net/rfc/deprecate_null_to_scalar_internal_arg

The Requests_Transport_fsockopen::request() method passes $request_body into the PHP native strlen() function without sufficient validation, which on PHP 8.1 results in a strlen(): Passing null to parameter #1 ($string) of type string is deprecated notification.

The existing RequestsTest_Transport_Base::testEmptyPOST() test method exposed this.

When investigating this issue, I realized that no significant input validation was being done on the $data parameter.
PR #368 in response to issue #248 added a test to verify that the "content-length" header was correctly set when null would be passed as $data and added a safe-guard specifically for when null would be passed. Also, as a matter of form, integers and floats were handled correctly for fsockopen, but anything else would lead to PHP errors in unexpected places in every supported PHP version, including integers and floats in combination with the cURL class.

To mitigate this, I propose to:

• Add a Requests_Exception_InvalidArgument class which extends the PHP native InvalidArgumentException.
• Add input validation for the $data parameter to both the cURL as well as the fsockopen Transport class which maintains and stabilizes the pre-existing behaviour for handling of null, synchronizes the behaviour for integers and floats to be the same across fsockopen and cURL, but will throw the new InvalidArgument exception for any other type of unexpected input for the $data parameter.

Includes additional tests covering this change.