`Content-Length` header not specified for empty POST requests with fsockopen transport

[dd32]

A reported to WordPress.org Trac in #38427 the fsockopen transport fails to send a Content-Length: 0 header for empty payloads.

This causes some servers to respond with a 411 error.
To quote the above ticket:

HTTP result code 411 is "Length required". RFC2616 Section 14.13 says Content-Length header "SHOULD" be included. In practice, it is not used for GET nor HEAD requests, but is expected for POST requests.