Set `Content-Length` header for OPTIONS requests

Note also that this change was previously suggested as part of the resolution of https://github.com/nodejitsu/node-http-proxy/pull/338, but it never made it to `master`.

I'm not so happy about special-casing individual HTTP verbs like this, and I doubt the maintainers are, either. A better solution would involve disabling the default Node.js behavior described in the commit message (below). That behavior might make sense to make the `http` and `https` modules more usable, but in the setting of an HTTP proxy, fidelity is much more important.

This seems possible via [the `useChunkedEncodingByDefault` attribute](https://github.com/joyent/node/blob/cfcb1de130867197cbc9c6012b7e84e08e53d032/lib/_http_outgoing.js#L275-L280), but that is undocumented and likely unstable. I wasn't able to get it working locally, likely because [this library uses `pipe` for *all* "incoming" requests](https://github.com/nodejitsu/node-http-proxy/blob/aba505d159a7dd46e46dd3ceb1b9565308b4d6af/lib/http-proxy/passes/web-incoming.js#L142). When piping data, the `useChunkedEncodingByDefault` flag does not seem to be honored. This probably makes sense because a streaming interface would generally emit data in chunks... so maybe special-casing HTTP verbs is the best approach, after all.

Commit message:

> Web browsers automatically issue an OPTIONS request in advance of other
> HTTP requests when the document's domain does not match the target in
> order to facilitate Cross-Origin Resource Sharing. These requests are
> forbidden from specifying a body and typically do not specify an
> (unecessary) `Length` header.
> 
> Node.js automatically adds the `Content-Encoding: chunked` header value
> to requests that do not specify a `Length` header. This makes proxied
> OPTIONS requests from web browsers invalid.
>
> Explicitly set the `Content-Length` header of all `OPTIONS` requests to
> "0", disabling the default "chunked" encoding behavior [2].
> 
> [1] http://www.w3.org/TR/cors/
> [2] http://nodejs.org/api/http.html