Modified TrueHeaders class to order headers

This issue was automatically migrated from github issue https://github.com/TheTorProject/ooni-probe/issues/298.

Fixes Issue 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.