Cookie\Jar tests: stabilize and improve

[jrfnl]

Cookie\Jar\GetIteratorTest: add message param to assertion

... as the same assertion is called multiple times.

Cookie\Jar\ArrayAccessTest: various minor improvements

• Add $message parameter to all assertions.
• Add extra assertion to the testCookieJarSetter() test verifying the cookie value was set correctly.
  As it was, if the value setting failed on both objects, the test would still have passed, which is not the intention.
• Use more specific assertions in the testCookieJarUnsetter() test.
  The value returned for array access to an unset value was tested with assertEmpty(). In reality, the code is specifically set to return null for unset/not set values, so let's make the test more precise and test for null.
  Along the same lines, we can use the more specific assertArrayNotHasKey() assertion instead of doing an isset() within an assertion.

Cookie\Jar\JarTest: stabilize tests

• Add $message parameter to all assertions.
• Add an additional assertion verifying that the request returned a Response instance.
• Add an additional assertion checking that $data['cookies'] is actually an array to the setCookieRequest() test helper method.

Cookie\Jar\JarTest: add extra test

.. to increase path coverage.

Related to #497