Verify Content-Type from smart HTTP servers

Before parsing a suspected smart-HTTP response verify the returned
Content-Type matches the standard. This protects a client from
attempting to process a payload that smells like a smart-HTTP
server response.

JGit has been doing this check on all responses since the dawn of
time. I mistakenly failed to include it in git-core when smart HTTP
was introduced. At the time I didn't know how to get the Content-Type
from libcurl. I punted, meant to circle back and fix this, and just
plain forgot about it.

Signed-off-by: Shawn Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/http-push.c b/http-push.c
index 8701c12..ba45b7b 100644
--- a/http-push.c
+++ b/http-push.c
@@ -1560,7 +1560,7 @@
 
 	sprintf(url, "%s%s", repo->url, path);
 
-	switch (http_get_strbuf(url, NULL, 0)) {
+	switch (http_get_strbuf(url, NULL, NULL, 0)) {
 	case HTTP_OK:
 		ret = 1;
 		break;
@@ -1584,7 +1584,7 @@
 	url = xmalloc(strlen(repo->url) + strlen(path) + 1);
 	sprintf(url, "%s%s", repo->url, path);
 
-	if (http_get_strbuf(url, &buffer, 0) != HTTP_OK)
+	if (http_get_strbuf(url, NULL, &buffer, 0) != HTTP_OK)
 		die("Couldn't get %s for remote symref\n%s", url,
 		    curl_errorstr);
 	free(url);