…cURL)`

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.