fixed the bug from issue #132 from original jieba issue track

- issue details from https://github.com/fxsjy/jieba/issues/132
 - updated tests
 - also some code refactors

posseg/viterbi.go

@@ -86,6 +86,11 @@ func Viterbi(obs []rune) (float64, []StateTag) {
 				obs_states = append(obs_states, tmp_obs_states[index])
 			}
 		}
+		if len(obs_states) == 0 {
+			for key := range prev_states_expect_next {
+				obs_states = append(obs_states, key)
+			}
+		}
 		if len(obs_states) == 0 {
 			obs_states = ProbTransKeys
 		}