http: write last header line late

http: write last header line late

- HEADERFUNCTIONS might inspect response properties like
  CURLINFO_CONTENT_LENGTH_DOWNLOAD_T on seeing the last header line. If
  the line is being written before this is initialized, values are not
  available.

- write the last header line late when analyzing a HTTP response so that
  all information is available at the time of the writing.

- add test1485 to verify that CURLINFO_CONTENT_LENGTH_DOWNLOAD_T works
  on seeing the last header.

Fixes #13752
Reported-by: Harry Sintonen
Closes #13757