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.

lib/http.c

@@ -2408,20 +2408,19 @@ CURLcode Curl_http(struct connectdata *conn, bool *done)
         data->set.postfieldsize:
         (data->set.postfields? (curl_off_t)strlen(data->set.postfields):-1);
     }
-    if(!data->req.upload_chunky) {
-      /* We only set Content-Length and allow a custom Content-Length if
-         we don't upload data chunked, as RFC2616 forbids us to set both
-         kinds of headers (Transfer-Encoding: chunked and Content-Length) */
 
-      if(conn->bits.authneg || !Curl_checkheaders(data, "Content-Length:")) {
-        /* we allow replacing this header if not during auth negotiation,
-           although it isn't very wise to actually set your own */
-        result = Curl_add_bufferf(req_buffer,
-                                  "Content-Length: %" CURL_FORMAT_CURL_OFF_T
-                                  "\r\n", postsize);
-        if(result)
-          return result;
-      }
+    /* We only set Content-Length and allow a custom Content-Length if
+       we don't upload data chunked, as RFC2616 forbids us to set both
+       kinds of headers (Transfer-Encoding: chunked and Content-Length) */
+    if((postsize != -1) && !data->req.upload_chunky &&
+       !Curl_checkheaders(data, "Content-Length:")) {
+      /* we allow replacing this header if not during auth negotiation,
+         although it isn't very wise to actually set your own */
+      result = Curl_add_bufferf(req_buffer,
+                                "Content-Length: %" CURL_FORMAT_CURL_OFF_T
+                                "\r\n", postsize);
+      if(result)
+        return result;
     }
 
     if(!Curl_checkheaders(data, "Content-Type:")) {