http_server: add an accessor to read request header

Signed-off-by: Shizuo Fujita <[email protected]>

Author: Watson1978

lib/fluent/plugin_helper/http_server/request.rb

@@ -29,6 +29,10 @@ def initialize(request)
           @path, @query_string = path.split('?', 2)
         end
 
+        def headers
+          @request.headers
+        end
+
         def query
           @query_string && CGI.parse(@query_string)
         end