This can be useful in many cases but I think sometimes it can lead to unexpected behaviors such as you first set header Content-Length with value 20, then you set body (with width 50) and value of Content-Length becomes 50, which is not the intent of the user.

In my humble opinion implementing higher level abstraction just for sending post requests would be a much better solution. Something like post fn taking uri and body as parameters.

Please let me know what you think and also please take a look at commit add post fn.

Set header Content-Length to the actual length of body is more common, in my opinion, and user can set it again if there is some special usecase, which may not be often