parser_json: fix wrong LoadError warning

If Oj is not installed, LoadError with the empty message is raised.
So, the current condition `/\boj\z/.match?(ex.message)` does not work
and the following meaningless warning is displayed.

    {datetime} [warn]: #x {id} LoadError

After this fix, the log message will be:

    {datetime} [info]: #x {id} Oj is not installed, and failing back to
    Yajl for json parser

Signed-off-by: Daijiro Fukuda <[email protected]>

lib/fluent/plugin/parser_json.rb

@@ -58,14 +58,14 @@ def configure_json_parser(name)
           raise "BUG: unknown json parser specified: #{name}"
         end
       rescue LoadError => ex
-        name = :yajl
         if log
-          if /\boj\z/.match?(ex.message)
+          if name == :oj
             log.info "Oj is not installed, and failing back to Yajl for json parser"
           else
             log.warn ex.message
           end
         end
+        name = :yajl
         retry
       end