http: make Content-Length parser more WHATWG

Return error if there is something after the number other than
whitespace and newline.

Allow comma separated numbers and repeated headers as long as the new value is
the same as was set before.

Add test 767 to 771 to verify.

Reported-by: Ignat Loskutov
Fixes #18921
Closes #18925

tests/data/test770

@@ -0,0 +1,58 @@
+<testcase>
+<info>
+<keywords>
+HTTP
+HTTP GET
+</keywords>
+</info>
+
+#
+# Server-side
+<reply>
+<data crlf="yes">
+HTTP/1.1 200 OK
+Date: Tue, 09 Nov 2010 14:49:00 GMT
+Server: test-server/fake
+Last-Modified: Tue, 13 Jun 2000 12:10:00 GMT
+ETag: "21025-dc7-39462498"
+Accept-Ranges: bytes
+Content-Length: 6,06,6
+Content-Length: 6,    6
+Connection: close
+Content-Type: text/html
+Funny-head: yesyes
+
+-foo-
+</data>
+</reply>
+
+#
+# Client-side
+<client>
+<server>
+http
+</server>
+<name>
+HTTP with Content-Length headers using comma separated list
+</name>
+<command>
+http://%HOSTIP:%HTTPPORT/%TESTNUMBER
+</command>
+</client>
+
+#
+# Verify data after the test has been "shot"
+<verify>
+<protocol crlf="yes">
+GET /%TESTNUMBER HTTP/1.1
+Host: %HOSTIP:%HTTPPORT
+User-Agent: curl/%VERSION
+Accept: */*
+
+</protocol>
+<limits>
+Allocations: 135
+Maximum allocated: 136000
+</limits>
+</verify>
+</testcase>