A.5.1.4 Multi-instance response headers now supported

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A.5.1.4 Multi-instance response headers now supported

Some HTTP headers can appear multiple times (for example Set-Cookie) - the response handling has been modified to allow multiple instances of a header. `ResponseMixin.get_header()` now returns a list of strings, rather than just a string. The httpdapp module has also been updated to allow multiple instances of a header, keeping headers in a list rather than a dictionary.