reports: Balance.__init__ better handles multiple amounts of same currency.

This is something that should've happened with 3d704e286
but I didn't think of it at the time.

conservancy_beancount/reports/core.py

@@ -90,8 +90,10 @@ class Balance(Mapping[str, data.Amount]):
     ) -> None:
         if tolerance is None:
             tolerance = self.TOLERANCE
-        self._currency_map = {amount.currency: amount for amount in source}
         self.tolerance = tolerance
+        self._currency_map: Dict[str, data.Amount] = {}
+        for amount in source:
+            self._add_amount(self._currency_map, amount)
 
     def _add_amount(self,
                     currency_map: MutableMapping[str, data.Amount],

tests/test_reports_balance.py

@@ -81,6 +81,26 @@ def test_mixed_balance():
     assert not balance.is_zero()
     assert all(balance[key] == testutil.Amount(amt, key) for key, amt in amounts.items())
 
+def test_init_recurring_currency():
+    balance = core.Balance([
+        testutil.Amount(20),
+        testutil.Amount(40),
+        testutil.Amount(60, 'EUR'),
+        testutil.Amount(-80),
+    ])
+    assert balance
+    assert balance['EUR'] == testutil.Amount(60, 'EUR')
+    assert balance['USD'] == testutil.Amount(-20)
+
+def test_init_zeroed_out():
+    balance = core.Balance([
+        testutil.Amount(25),
+        testutil.Amount(40, 'EUR'),
+        testutil.Amount(-25),
+        testutil.Amount(-40, 'EUR'),
+    ])
+    assert balance.is_zero()
+
 @pytest.mark.parametrize('mapping,expected', [
     ({}, True),
     ({'USD': 0}, True),