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/http_fetcher.cc b/http_fetcher.cc
index 9e1e0ac..b59f988 100644
--- a/http_fetcher.cc
+++ b/http_fetcher.cc
@@ -17,11 +17,17 @@
   }
 }
 
-void HttpFetcher::SetPostData(const void* data, size_t size) {
+void HttpFetcher::SetPostData(const void* data, size_t size,
+                              HttpContentType type) {
   post_data_set_ = true;
   post_data_.clear();
   const char *char_data = reinterpret_cast<const char*>(data);
   post_data_.insert(post_data_.end(), char_data, char_data + size);
+  post_content_type_ = type;
+}
+
+void HttpFetcher::SetPostData(const void* data, size_t size) {
+  SetPostData(data, size, kHttpContentTypeUnspecified);
 }
 
 // Proxy methods to set the proxies, then to pop them off.