Cookie\Jar: add input validation

[jrfnl]

Cookie\Jar: move tests to dedicated file

The tests for the Cookie\Jar class were up to now mixed in with the tests for the Cookie class.

This moves the Jar tests to a dedicated test file.

The actual test code has not been touched.

This includes duplicating a helper method. At a later point in time when these tests are reviewed, it can be decided whether this helper should move to the TestCase.

Cookie\Jar::__construct(): add input validation

As array_merge() is used on the $cookies property in the before_redirect_check() method, only arrays can be accepted.

Includes test.

Cookie\Jar::normalize_cookie(): fix default value of $key

The $key parameter corresponds to the $name parameter in the Cookie::parse() method, which expects a string and has a default value of ''.

This fixes the default value of the $key parameter to be the same, i.e. an empty string.

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Regarding the other public methods:

• The public magic/ArrayIterator methods should not need input validation as they should not be called directly, but only indirectly and when called that way, will receive the correct input type.
• The normalize_cookie() method checks for an instance of Cookie already and if not passes off to Cookie::parse() for which input validation is being added in PR Cookie: add input validation #609
• The register() and before_redirect_check() methods already have a class based type declaration.
• The before_request() method is intended to be only called as a Hook callback.
• The other public methods do not take parameters.