Content-Length header has wrong value

[dskloet]

At https://github.com/SherClockHolmes/webpush-go/blob/master/webpush.go#L186
the Content-Length header is set to len(ciphertext).
This is the wrong value but it doesn't matter because somehow the net/http library sets the value to the actual length of the actual content.
I discovered this because I had the same mistake and couldn't get my code to work after porting it to PHP, until I set the Content-Length to recordSize.
When I sent my request to my own server instead of to the notification endpoint, I discovered that the actual value of the Content-Length header is different in Go vs. PHP.