Support number/string agnostic JSON decoding

amount.go

@@ -315,16 +315,22 @@ func (a Amount) MarshalJSON() ([]byte, error) {
 // UnmarshalJSON implements the json.Unmarshaler interface.
 func (a *Amount) UnmarshalJSON(data []byte) error {
 	aux := struct {
-		Number       string `json:"number"`
-		CurrencyCode string `json:"currency"`
+		Number       json.RawMessage `json:"number"`
+		CurrencyCode string          `json:"currency"`
 	}{}
 	err := json.Unmarshal(data, &aux)
 	if err != nil {
 		return err
 	}
+
+	var auxNumber string
+	if err = json.Unmarshal(aux.Number, &auxNumber); err != nil {
+		auxNumber = string(aux.Number)
+	}
+
 	number := apd.Decimal{}
-	if _, _, err := number.SetString(aux.Number); err != nil {
-		return InvalidNumberError{aux.Number}
+	if _, _, err := number.SetString(auxNumber); err != nil {
+		return InvalidNumberError{auxNumber}
 	}
 	if aux.CurrencyCode == "" || !IsValid(aux.CurrencyCode) {
 		return InvalidCurrencyCodeError{aux.CurrencyCode}

amount_test.go

@@ -812,6 +812,20 @@ func TestAmount_UnmarshalJSON(t *testing.T) {
 		t.Errorf("got %T, want currency.InvalidNumberError", err)
 	}
 
+	d = []byte(`{"number": {"key": "value"}, "currency": "USD"}`)
+	err = json.Unmarshal(d, unmarshalled)
+	if e, ok := err.(currency.InvalidNumberError); ok {
+		if e.Number != `{"key": "value"}` {
+			t.Errorf(`got %v, "want {"key": "value"}"`, e.Number)
+		}
+		wantError := `invalid number "{\"key\": \"value\"}"`
+		if e.Error() != wantError {
+			t.Errorf("got %v, want %v", e.Error(), wantError)
+		}
+	} else {
+		t.Errorf("got %T, want currency.InvalidNumberError", err)
+	}
+
 	d = []byte(`{"number":"3.45","currency":"usd"}`)
 	err = json.Unmarshal(d, unmarshalled)
 	if e, ok := err.(currency.InvalidCurrencyCodeError); ok {