Fix JSON schema for Xero Invoice

src/Xero.hs

@@ -180,7 +180,7 @@ data XeroInvoice =
     , xi_number :: T.Text
     , xi_contact :: T.Text
     , xi_currency :: T.Text
-    , xi_currRate :: Double
+    , xi_currRate :: Maybe Double
     , xi_total :: Double
     , xi_status :: T.Text
     , xi_date :: T.Text
@@ -199,7 +199,7 @@ instance FromJSON XeroInvoice where
       c <- obj .: "Contact"
       cN <- c .: "Name"
       cu <- obj .: "CurrencyCode"
-      cR <- obj .: "CurrencyRate"
+      cR <- obj .:? "CurrencyRate"
       total <- obj .: "AmountDue"
       status <- obj .: "Status"
       d <- obj .: "Date"