Multiple response headers

[gsnedders]

This is kinda the counter-part to #84.

Given a headers file containing:

Link: <support/cascade-import-002d.css>; rel="stylesheet"
Link: <support/cascade-import-002f.css>; rel="stylesheet"

This ends in the response headers being:

HTTP/1.1 200 OK
Content-Type: application/xhtml+xml
Link: <support/cascade-import-002f.css>; rel="stylesheet"
Server: BaseHTTP/0.3 Python/2.7.12
Date: Wed, 17 Aug 2016 16:56:16 GMT

i.e., only the last of the Link headers gets output.

While in theory Link: <support/cascade-import-002d.css>; rel="stylesheet", <support/cascade-import-002f.css>; rel="stylesheet" is equivalent, not all UAs have done this for all headers historically. As such, we should probably have some way of actually sending them in the response.