fix wrong use from Headers array

we have here the array index "Content-Length" and a value typed integer
[#4897] Fix condition

- If the CURLOPT_INFILESIZE option *is* available, but no content-length
  header is sent, we can proceed. Only reset the CURLOPT_INFILESIZE if
  the content-length header is present.
Merge branch 'hotfix/4897'

Close #4897