Set `Content-Length` header for OPTIONS requests

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
Merge pull request #742 from jugglinmike/option-content-length

Set `Content-Length` header for OPTIONS requests