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/t5550-http-fetch-dumb.sh

@@ -191,5 +191,10 @@ test_expect_success 'http error messages are reencoded' '
 	grep "this is the error message" stderr
 '
 
+test_expect_success 'reencoding is robust to whitespace oddities' '
+	test_must_fail git clone "$HTTPD_URL/error/odd-spacing" 2>stderr &&
+	grep "this is the error message" stderr
+'
+
 stop_httpd
 test_done