Preserve ordering of gathered headers

[ssokolow]

Websites may try to use the order in which headers are sent over the wire to distinguish real browsers from bots.

I'll want to switch from using dicts to using OrderedDicts and add some tests to ensure order is preserved all the way through the system.

[ssokolow]

I'll also need some way to retrieve the ordering of all headers (including those filtered out for being unsafe to blindly mimic) so it can be preserved when merging in things like Referer and the defaults from libraries like python-requests.