http: fix charset detection of extract_content_type()

extract_content_type() could not extract a charset parameter if the
parameter is not the first one and there is a whitespace and a following
semicolon just before the parameter. For example:

    text/plain; format=fixed ;charset=utf-8

And it also could not handle correctly some other cases, such as:

    text/plain; charset=utf-8; format=fixed
    text/plain; some-param="a long value with ;semicolons;"; charset=utf-8

Thanks-to: Jeff King <peff@peff.net>
Signed-off-by: Yi EungJun <eungjun.yi@navercorp.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/lib-httpd/error.sh

@@ -19,6 +19,10 @@ case "$PATH_INFO" in
 	printf "text/plain; charset=utf-16"
 	charset=utf-16
 	;;
+*odd-spacing*)
+	printf "text/plain; foo=bar ;charset=utf-16; other=nonsense"
+	charset=utf-16
+	;;
 esac
 printf "\n"