make cookie parsing case insensitive

[artemredkin]

Make cookie parsing case-insensitive, closes #205

Motivation:
As per https://tools.ietf.org/html/rfc6265#section-5.2.2, cookie names should be case-insensitive, and currently they are not.

Modifications:

1. always split cookie component into name and optional value, lowercase the name and then match by this name using pre-defined lowercased constant
2. modify one of the tests to send differently-cased cookie components

Result:
Cookie parsing is case-insensitive.

[artemredkin]

@swift-server-bot test this please

[vlm]

This code introduces a denial of service.

#209