buffet: add unit tests for http_utils

Added more unit tests for utility functions in http_utils.h

BUG=chromium:367379
TEST=Old and new unit tests pass.

Change-Id: I04e28691bc4db3980783c0033f5691087a8c6ab3
Reviewed-on: https://chromium-review.googlesource.com/197312
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/buffet/http_transport_fake.h b/buffet/http_transport_fake.h
index c526770..b95599f 100644
--- a/buffet/http_transport_fake.h
+++ b/buffet/http_transport_fake.h
@@ -81,6 +81,9 @@
   // Add/retrieve request/response HTTP headers.
   void AddHeaders(const HeaderList& headers);
   std::string GetHeader(const std::string& header_name) const;
+  const std::map<std::string, std::string>& GetHeaders() const {
+    return headers_;
+  }
 
  protected:
   // Data buffer.