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

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

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