Add unit test for `Tags.validateString'.
Add unit test for `Tags.validateString'.

This is to follow up on issue #8.

Change-Id: I5c190137d66dac6716303e006733fdc2545e615c

eswdd/opentsdb,eswdd/opentsdb,kidaa/opentsdb,OpenTSDB/opentsdb,ShefronYudy/opentsdb,johann8384/opentsdb,MadDogTechnology/opentsdb,ShefronYudy/opentsdb,alienth/opentsdb,johann8384/opentsdb,andyflury/opentsdb,MadDogTechnology/opentsdb,kidaa/opentsdb,sidhhu/opentsdb,andyflury/opentsdb,OpenTSDB/opentsdb,OpenTSDB/opentsdb,alienth/opentsdb,sidhhu/opentsdb,MadDogTechnology/opentsdb