POST request without content-length possible?

[FrederikWolter]

Hi,
I am trying to optimize my code running on an ESP8266, which uploads the measurements of a BME280.
A ineffective section, I think, is the calculation of "Content-Length", so I want to change it.

client.print(F("POST /boxes/"));
printHexToStream(SENSEBOX_ID, 12, client);
client.println(F("/data HTTP/1.1"));

// print once to Serial to get the content-length
int contentLen = printCsvToStream(Serial);

// Send the required header parameters
client.print(F("Host: "));
client.println(server);
client.println(F("Content-Type: text/csv\nConnection: close"));
client.print(F("Content-Length: "));
client.println(contentLen);
client.println();
printCsvToStream(client);
client.println();

client.flush();
client.stop();`

Expected Behavior

I tried leaving the "Content-Length" header out with the manual POST-request tool Postman and it worked!

POST /boxes/5cc2cfb9facf70001ac953f6/data HTTP/1.1
Host: ingress.opensensemap.org
Content-Type: text/csv
Connection: close
Cache-Control: no-cache
Postman-Token: 50a6222f-181e-ead7-4cba-a79b9aeaef95

5cc2cfb9facf70001ac953fb,28.87
5cc2cfb9facf70001ac953fa,43.66
5cc2cfb9facf70001ac953f9,1006.28

Actual Behavior

But using the ESP the measurements don´t update.

What is my mistake?
Is it not possible to leave the length out? How can I optimize this instead?