improbe DEFAULTADDRESS and DEFAULTPORT behavior

I have add some feature for mruby-growthforecast.
Would you please merge this patch?

* some typo fixed
* skippable address like below.  
`GrowthForecast::Client.new()` will be 127.0.0.1:5125
* skippable port like below.  
`GrowthForecast::Client.new('10.100.0.11')` will be 10.100.0.11:5125