Support multiple identical header fields in http responses "On Air"

[rodja]

While working on #1603 it was discovered that we do not yet allow multiple 'set-cookie' entries in the header fields. This PR fixes that by transferring the response headers as list of tuples instead a dict.

[falkoschindler]

@rodja I'm not sure if this does exactly what you intended to. Your comment sounded like you're expecting the headers with duplicate keys to be transmitted as a list of tuples like

[('a', '1'), ('b', '2'), ('c', '3'), ('c', '42')]

But instead items() generates tuples like

[('a', '1'), ('b', '2'), ('c', '3, 42')]

I'm not yet sure if this works with the server implementation.

[rodja]

Thanks @falkoschindler. We need to use the .raw property to get all items:

import httpx

headers = httpx.Headers([(b'test', b'A'), (b'test', b'B')])
print([(k.decode(), v.decode()) for k, v in headers.raw])

I've already pushed the update to this PR.

[falkoschindler]

headers.raw was still wrong. I guess you meant response.headers.raw.
But we can use response.headers.multi_items() instead, which does the list comprehension and decoding for us.