fix: correct spelling and add processing of observed_area

Author: uwzm

frost_sta_client/model/multi_datastream.py

@@ -310,7 +310,8 @@ def __setstate__(self, state):
         self.name = state.get('name', None)
         self.description = state.get('description', None)
         self.observation_type = state.get('observationType', None)
-        self.observation_area = state.get('observedArea', None)
+        if state.get('observedArea', None) is not None:
+            self.observed_area = frost_sta_client.utils.process_area(state['observedArea'])
         self.phenomenon_time = state.get('phenomenonTime', None)
         self.result_time = state.get('resultTime', None)
         self.properties = state.get('properties', None)