econdb: fix bug with dataset requests

handle cases where we receive series with differing numbers of metadata dimensions

Author: StochasticDoge

pandas_datareader/econdb.py

@@ -90,10 +90,13 @@ def show_func(x):
             def show_func(x):
                 return x[: x.find(":")]
 
+        unique_keys = {k for s in results for k in s['additional_metadata']}
         for entry in results:
             series = pd.DataFrame(entry["data"])[["dates", "values"]].set_index("dates")
             head = entry["additional_metadata"]
-
+            for k in unique_keys:
+                if k not in head:
+                    head[k] = '-1:None'
             if head != "":  # this additional metadata is not blank
                 series.columns = pd.MultiIndex.from_tuples(
                     [[show_func(x) for x in head.values()]],