initialize argument error

After fluentd 0.10.49, fluent-plugin-http-ex 0.0.2 raises ArgumentError again. The reason is Hander#initialize(in original in_http.rb) arguments was increased.

* https://github.com/fluent/fluentd/blob/v0.10.48/lib/fluent/plugin/in_http.rb#L156
* https://github.com/fluent/fluentd/blob/v0.10.49/lib/fluent/plugin/in_http.rb#L195

To fix this, fluent-plugin-http-ex should support format argument.