history: normalize insertion dates from the db to utc

Since straight out of the db we only get an offset,
and the utc timezone is quite implicit. We want it to be explicit
since pandas can change it interpretation from version to version
(e.g. .23 to .24 in a .loc in the presence of a datetime with offset
but no tz).

(we know it is utc because:

 * postgres is configured (see conftest) to UTC
 * we insert as utc
)

tshistory/tsio.py

@@ -222,6 +222,11 @@ class timeseries(SeriesServices):
         if not revs:
             return {}
 
+        revs = [
+            (csid, pd.Timestamp(idate).astimezone('UTC'))
+            for csid, idate in revs
+        ]
+
         if diffmode:
             # compute the previous serie value
             first_csid = revs[0][0]
@@ -270,10 +275,7 @@ class timeseries(SeriesServices):
                  for idate, ts in series
             ]
 
-        return {
-            pd.Timestamp(idate).astimezone('UTC'): serie
-            for idate, serie in series
-        }
+        return dict(series)
 
     @tx
     def staircase(self, cn, seriename, delta,