http: OPTIONS shouldn't default to chunked encoding

OPTIONS requests have an optional request body, but Node's http client currently adds a `Transfer-Encoding: chunked` header to all OPTIONS requests. This header indicates that a request body is present even if there is no request body. This subsequently confuses some servers and causes requests to fail.

This same issue cropped up for [DELETE requests](https://github.com/joyent/node/issues/6185), and this patch simply applies that same fix for OPTIONS requests.

For a concrete example of the current behavior's impact, using node-http-proxy in conjunction with a Varnish backend server will break the OPTIONS requests from CORS preflight requests. This is because CORS preflight OPTIONS requests don't have a body and Varnish doesn't like `Transfer-Encoding: chunked` accompanied by an empty body.

And finally, just from [the spec](http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html#sec9.2) indicating the request body is optional (and currently undefined) for OPTIONS requests:

> If the OPTIONS request includes an entity-body (as indicated by the presence of Content-Length or Transfer-Encoding), then the media type MUST be indicated by a Content-Type field. Although this specification does not define any use for such a body, future extensions to HTTP might use the OPTIONS body to make more detailed queries on the server. A server that does not support such an extension MAY discard the request body.