Server API recommendation: auto set content length from body provided

[jentfoo]

It's awkward and confusing to have to know that you implicitly need to add the content length header. This is easier to do in the client right now since the body content is provided before the request is made.

However for the HTTPServer's ResponseWriter we specify the body separate from the headers. This may be a harder problem to solve (but we could potentially add some API sugar when the entire content is known before hand).