http, write last header line late

[icing]

• 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.
• refs #13752