Bugfix: fix mishandling of POST/PUT requests with empty body, causing net/http to send request without a Content-Length header

[manicminer]

This PR addresses a mishandling of POST and PUT requests having an empty body.

In a few places within net/http, in order to determine whether the request has a body or not, Request.Body which is an io.ReadCloser, does not have its Read() method called but is instead simply nil checked or compared to the http.NoBody value - so even if its Read() method returns 0, io.EOF, it's still considered to represent a nonempty request body.

// NoBody is an io.ReadCloser with no bytes. Read always returns EOF
// and Close always returns nil. It can be used in an outgoing client
// request to explicitly signal that a request has zero bytes.
// An alternative, however, is to simply set Request.Body to nil.
var NoBody = noBody{}

type noBody struct{}

func (noBody) Read([]byte) (int, error)         { return 0, io.EOF }
func (noBody) Close() error                     { return nil }
func (noBody) WriteTo(io.Writer) (int64, error) { return 0, nil }

(Note this docstring also suggests that you should set Request.Body to nil or http.NoBody)

For HTTP/1.1, even when the request body is an io.ReadCloser with a zero length buffer, the resulting request on the wire then contains neither a Content-Length nor a Transfer-Encoding header and is therefore invalid (incurring a 411 response from a compliant server).

For HTTP/2.0, the http.http2actualContentLength() function, if it isn't specifically http.NoBody, the Content-Length is incorrectly discarded instead of being set to zero, which has the same effect.