curlhttpsink: update HTTP header for curl 7.66

Change how content-length is set for HTTP POST headers, letting curl automatically set the header (given the content-length) instead of manually writing it. This enables curl to know the content-length and automatically set that field in the header. In curl 7.66 (and later), curl will use Transfer-Encoding: chunked by default if curl does not know the content-length (e.g. when manually writing the header). This results in a header that has both a content-length and a transfer-encoding, which is not allowed in HTTP/1.1.