Allow using both setHeader and resp w/ headers

[iffy]

Currently making a request of a handler like this will only include the Content-Type header. This change makes it so that the foo and bar headers are added to the response as well.

  get "/manyheaders":
    setHeader(responseHeaders, "foo", "foo")
    setHeader(responseHeaders, "bar", "bar")
    resp Http200, {"Content-Type": "text/plain"}, "result"

[iffy]

Oops; this includes the code from #267 as well. I can remake this PR if you'd like.

[dom96]

Yeah, please split it if you can.

[dom96]

Any news on this? :)

[iffy]

I'll wait for #275 and then remake this when I can get some time

[dom96]

Friendly ping

[iffy]

I've submitted #300 to do the minimal change.