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/omaha_request_action.cc b/omaha_request_action.cc
index 6311332..bbcc00b 100644
--- a/omaha_request_action.cc
+++ b/omaha_request_action.cc
@@ -233,7 +233,8 @@
                                     ping_active_days_,
                                     ping_roll_call_days_,
                                     prefs_));
-  http_fetcher_->SetPostData(request_post.data(), request_post.size());
+  http_fetcher_->SetPostData(request_post.data(), request_post.size(),
+                             kHttpContentTypeTextXml);
   LOG(INFO) << "Posting an Omaha request to " << params_.update_url;
   LOG(INFO) << "Request: " << request_post;
   http_fetcher_->BeginTransfer(params_.update_url);