[tor-bugs] #11977 [Ooni]: Modified TrueHeaders class to order headers

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#11977: Modified TrueHeaders class to order headers
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 Reporter:  cypherpunks  |          Owner:  hellais
     Type:  defect       |         Status:  new
 Priority:  normal       |      Milestone:
Component:  Ooni         |        Version:
 Keywords:               |  Actual Points:
Parent ID:               |         Points:
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 This issue was automatically migrated from github issue
 https://github.com/TheTorProject/ooni-probe/issues/298.

 Fixes [Issue 254](https://github.com/TheTorProject/ooni-probe/issues/254)
 by storing TrueHeaders in an `OrderedDict`.

 Removes clutter from TrueHeaders class by making `_rawHeaders` a simple
 `OrderedDict` mapping from header string to list of value strings. This
 conforms with its type in the superclass
 (`twisted.web.http_headers.Headers`) and removes the need to override a
 few methods. A new dict `_headerCases` stores the mapping from lowercase
 header name to their original capitalization. This is accessed by
 overriding the superclass method `Headers._canonicalNameCaps`.

 Adds `test_trueheaders.test_order_preserved` to ensure `getAllRawHeaders`
 returns headers in the order they were set.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/11977>
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