Add test for Era.ToString()
Add test for Era.ToString()

This was previously implicitly being called by test discovery.

malcolmr/nodatime,BenJenkinson/nodatime,BenJenkinson/nodatime,malcolmr/nodatime,nodatime/nodatime,jskeet/nodatime,malcolmr/nodatime,jskeet/nodatime,nodatime/nodatime,malcolmr/nodatime