Update engine properly sets Content-Type to text/xml.

Since the payload sent to Omaha is XML encoded, update engine should
properly set the HTTP Content-Type header to text/xml, instead of using
the libcurl default (application/x-www-form-urlencoded).

BUG=chromium-os:7613
TEST=Tested against devserver, ensuring that Content-Type is set
correctly

Change-Id: I9766e8dd67ffd387634a0ab4ef83c2990b16b537
Reviewed-on: https://gerrit.chromium.org/gerrit/16051
Reviewed-by: Andrew de los Reyes <adlr@chromium.org>
Tested-by: Gilad Arnold <garnold@chromium.org>
Commit-Ready: Gilad Arnold <garnold@chromium.org>
diff --git a/libcurl_http_fetcher.h b/libcurl_http_fetcher.h
index e87c678..4fc65b2 100644
--- a/libcurl_http_fetcher.h
+++ b/libcurl_http_fetcher.h
@@ -29,6 +29,7 @@
       : HttpFetcher(proxy_resolver),
         curl_multi_handle_(NULL),
         curl_handle_(NULL),
+        curl_http_headers_(NULL),
         timeout_source_(NULL),
         transfer_in_progress_(false),
         transfer_size_(0),
@@ -191,6 +192,7 @@
   // Handles for the libcurl library
   CURLM *curl_multi_handle_;
   CURL *curl_handle_;
+  struct curl_slist *curl_http_headers_;
 
   // Lists of all read(0)/write(1) file descriptors that we're waiting on from
   // the glib main loop. libcurl may open/close descriptors and switch their