set default_codec doc attribute

Author: jsvd

docs/index.asciidoc

@@ -1,5 +1,6 @@
 :plugin: monasca_log_api
 :type: output
+:default_codec: json
 
 ///////////////////////////////////////////
 START - GENERATED VARIABLES, DO NOT EDIT!
@@ -176,4 +177,6 @@ monasca-log-api configuration
 
 
 [id="plugins-{type}s-{plugin}-common-options"]
-include::{include_path}/{type}.asciidoc[]
+include::{include_path}/{type}.asciidoc[]
+
+:default_codec!:

logstash-output-monasca_log_api.gemspec

@@ -1,6 +1,6 @@
 Gem::Specification.new do |s|
   s.name = 'logstash-output-monasca_log_api'
-  s.version         = '1.0.2'
+  s.version         = '1.0.3'
   s.licenses = ['Apache-2.0']
   s.summary = 'This gem is a logstash output plugin to connect via http to monasca-log-api.'
   s.description = 'This gem is a logstash plugin required to be installed on top of the Logstash core pipeline using $LS_HOME/bin/plugin install gemname. This gem is not a stand-alone program'