Remove duplication of partitionKey that resulted in invalid json

[igvk]

Cookies having Partition Keys are serialized incorrectly leading to errors when sending such cookies to the browser due to invalid json.

[kblok]

Do you wan to add some tests?

[igvk]

Yeah, it's a pity there are no tests on cookie partitioning. I'll see what I can do.

[kblok]

Yeah, it's a pity there are no tests on cookie partitioning.

I'll see what I can do.

At least testing the de/serialization

[igvk]

Here is the test of partition cookie