Correct false positives in language determination.

[shibacow]

HI
twitter.user.lang has become null due to a sudden specification change of twitter streaming api.

https://developer.twitter.com/en/docs/tweets/sample-realtime/api-reference

so,now.input parameter is not work.

 lang                ja,en                    # Optional

in_twitter.rb return allways true,even a language different from the specified language.

 return false if (!@lang.nil? && @lang != '') && !@lang.include?(tweet.user.lang)

so.therefore, I changed to use twitter.lang instead of twitter.user.lang to determine the language.