Add Converged to infinite with negative condition (#3)

* Add Converted to negative condition

* Fix test

* Decrease positive condition to 1 million

* Correct comparisson

* Revert test change

lib/ex_xirr.ex

@@ -182,7 +182,10 @@ defmodule ExXirr do
   @spec calculate(atom(), list(), float(), float(), integer()) ::
           {:ok, float()} | {:error, String.t()}
 
-  defp calculate(:xirr, _, _, rate, _) when rate > 1_000_000_000,
+  defp calculate(:xirr, _, _, rate, _) when rate > 1_000_000,
+    do: {:error, "Converged on infinity."}
+
+  defp calculate(:xirr, _, _, rate, _) when rate < -1_000_000,
     do: {:error, "Converged on infinity."}
 
   defp calculate(:xirr, _, acc, rate, _) when acc in [-0.0, +0.0], do: {:ok, Float.round(rate, 8)}