Use #to_s for Hash object

Hash does not have `#bytesize` method.

Author: okkez

lib/fluent/logger/fluent_logger.rb

@@ -176,7 +176,7 @@ def connection_string
 
       def pending_bytesize
         if @pending
-          @pending.bytesize
+          @pending.to_s.bytesize
         else
           0
         end