Fix AU fallback proxy mechanism for resumed updates.

This fixes a subtle problem where resuming an update in the presence of
a failing proxy would consistently fail, without resorting to
alternative proxies or a direct connection.

BUG=chromium-os:29042
TEST=Unittests; tested resume update with bogus proxy configuration.

Change-Id: I81eab8cf65451c891569091eda84ce9a18a7f8ab
Reviewed-on: https://gerrit.chromium.org/gerrit/20119
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: Darin Petkov <petkov@chromium.org>
Reviewed-by: Andrew de los Reyes <adlr@chromium.org>
Commit-Ready: Gilad Arnold <garnold@chromium.org>
Tested-by: Gilad Arnold <garnold@chromium.org>
diff --git a/libcurl_http_fetcher.cc b/libcurl_http_fetcher.cc
index abc4389..e1e93cb 100644
--- a/libcurl_http_fetcher.cc
+++ b/libcurl_http_fetcher.cc
@@ -217,6 +217,7 @@
   no_network_retry_count_ = 0;
   http_response_code_ = 0;
   terminate_requested_ = false;
+  sent_byte_ = false;
   ResumeTransfer(url_);
   CurlPerformOnce();
 }