HTTP POST: omit Content-Length if data size is unknown

This prevents sending a `Content-Length: -1` header, e.g this ocurred
with the following combination:

* standard HTTP POST (no chunked encoding),
* user-defined read function set,
* `CURLOPT_POSTFIELDSIZE(_LARGE)` NOT set.

With this fix it now behaves like HTTP PUT.
test1514: added - no more negative Content-Length (HTTP POST)

This covers changes from commit afd288b2.