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

Set `Content-Length` header for OPTIONS requests