Multi-value tornado headers are fetched with get() rather than get_list() #1109

In the tornado parser, we have multidict support for headers which gets data using get(...). This combines headers, and therefore gives wrong results.
Given the setup:

>>> import tornado
>>> h = tornado.httputil.HTTPHeaders()
>>> h.add("a", "foo")
>>> h.add("a", "bar")

Compare:

>>> h.get("a")
'foo,bar'
>>> h.get_list("a")
['foo', 'bar']

We should add a regression test for this and a fix.

This was flagged in #1108, which was summarily closed as a low-quality PR in violation of our AI policy.