Make sure we handle failures in mediaproxied requests gracefully

[tadzik]

Fixes matrix-org/matrix-appservice-irc#1816 blowing up the whole bridge when the request fails for any reason.

[Half-Shot]

We can do a bit better :)

[Half-Shot]

I suggest something like:

            if (getRes.headers["content-disposition"]) {
                res.setHeader("content-disposition", getRes.headers["content-disposition"] as string);
            }
            if (getRes.headers["content-type"]) {
                res.setHeader("content-type", getRes.headers["content-type"] as string);
            }
            if (getRes.headers["content-length"]) {
                res.setHeader("content-length", getRes.headers["content-length"] as string);
            }

which prevent the undefined values from throwing here.

[tadzik]

And now we can drop type casts too. The compiler was right all along :)