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

tests/data/test1485

@@ -0,0 +1,51 @@
+<testcase>
+<info>
+<keywords>
+HTTP
+HTTP GET
+</keywords>
+</info>
+
+# Server-side
+<reply>
+<data>
+HTTP/1.1 200 OK
+Server: Someone
+Content-Length: 7
+
+123456
+</data>
+<datacheck>
+HTTP/1.1 200 OK
+Server: Someone
+Content-Length: 7
+
+123456
+</datacheck>
+</reply>
+# Client-side
+<client>
+<server>
+http
+</server>
+<tool>
+lib%TESTNUMBER
+</tool>
+<name>
+get curlinfo on last header in callback
+</name>
+<command>
+http://%HOSTIP:%HTTPPORT/%TESTNUMBER
+</command>
+</client>
+
+# Verify data after the test has been "shot"
+<verify>
+<protocol>
+GET /%TESTNUMBER HTTP/1.1
+Host: %HOSTIP:%HTTPPORT
+Accept: */*
+
+</protocol>
+</verify>
+</testcase>