Fixes #94; keep case of chunk when linking username
Fixes #94; keep case of chunk when linking username

Because `name` and `chunk` can be references to the same object,
the destructive `name.downcase!` also affects `chunk`. This patch
ensures that the two are never the same object, and avoids the
unintended side-effect.

joebos/twitter-text-py,masonkmeyer/twitter-text,joebos/twitter-text-py,joebos/twitter-text-py,zhaoguohui/twitter-text,zhaoguohui/twitter-text,aaronsakowski/twitter-text,zhaoguohui/twitter-text,megaserg/twitter-text,masonkmeyer/twitter-text,zhaoguohui/twitter-text,twitter/twitter-text,megaserg/twitter-text,megaserg/twitter-text,iadvize/javascript-twitter-text-library,megaserg/twitter-text,joebos/twitter-text-py,zhaoguohui/twitter-text,aaronsakowski/twitter-text,aaronsakowski/twitter-text,aaronsakowski/twitter-text,ay8s/twitter-text,iadvize/javascript-twitter-text-library,ay8s/twitter-text,iadvize/javascript-twitter-text-library,twitter/twitter-text,twitter/twitter-text,silentroach/twitter-text,ay8s/twitter-text,silentroach/twitter-text,megaserg/twitter-text,masonkmeyer/twitter-text,twitter/twitter-text,silentroach/twitter-text,silentroach/twitter-text,aaronsakowski/twitter-text,masonkmeyer/twitter-text,joebos/twitter-text-py,ay8s/twitter-text,joebos/twitter-text-py,silentroach/twitter-text,twitter/twitter-text,masonkmeyer/twitter-text,ay8s/twitter-text,iadvize/javascript-twitter-text-library,iadvize/javascript-twitter-text-library