Workaround for string header keywords.

[dtenny]

This fixes Hunchensocket so that it does not break when Chunga returns string keys for header key/value pairs.

See #45
and edicl/chunga#13

IMO it would have been better to address the issue in Chunga than Hunchensocket, as the chunga change may have broken any number of upstream callers. But for now this was just something I did to un-break my web app, as the Chunga response was "work around it".

Whether people notice the breakage may depend on the headers sent by their respective browsers/platforms. For me, as noted in a code comment, there were string headers for the following headers: "DNT" "Sec-GPC" "Sec-Fetch-Dest" "Sec-Fetch-Mode" "Sec-Fetch-Site"

[dkochmanski]

I've proposed a change to chunga that addresses this issue:

• read-http-headers no longer interns all headers edicl/chunga#14
• read-http-headers skips unknown headers from the result edicl/chunga#15

most notably WS headers are added to the list, and unknown headers are not put at all in headers-in.