Make chunked transfer-encoding optional

[gitblit]

By flushing the response buffer we force chunked transfer-encoding. When we do this we lose the option to relay Content-Length to the client for HEAD requests or for setting-up client-side deterministic download progress indicators on GET requests.

This PR makes chunked transfer encoding optional and improves the HEAD response of the DirectoryHandler.

[coveralls]

Coverage decreased (-0.03%) to 11.023% when pulling bafb890 on chunked into c024a56 on master.

[decebals]

Thanks!