fix nunique (#341)

* fix nunique

* fix Series.nunique

Author: Rubtsowa

sdc/datatypes/hpat_pandas_series_functions.py

@@ -2896,11 +2896,12 @@ def hpat_pandas_series_nunique_str_impl(self, dropna=True):
             It is better to merge with Numeric branch
             """
 
-            str_set = set(self._data)
-            if dropna == False:
-                return len(str_set) - 1
-            else:
-                return len(str_set)
+            data = self._data
+            if dropna:
+                nan_mask = self.isna()
+                data = self._data[~nan_mask._data]
+            unique_values = set(data)
+            return len(unique_values)
 
         return hpat_pandas_series_nunique_str_impl