prices: Use default seat filler of 2

Discovered while using ubers, if a seat count is
void so 0, Uber expects the default to be set to 2.

Author: odeke-em

v1/prices.go

@@ -242,6 +242,8 @@ func (upf *UpfrontFare) NoCarsAvailable() bool {
 
 var errInvalidSeatCount = errors.New("invalid seatcount, default and maximum value is 2")
 
+const defaultSeatCount = 2
+
 func (esReq *EstimateRequest) validateForUpfrontFare() error {
 	if esReq == nil {
 		return errNilEstimateRequest
@@ -253,6 +255,10 @@ func (esReq *EstimateRequest) validateForUpfrontFare() error {
 		return errInvalidSeatCount
 	}
 
+	if esReq.SeatCount == 0 {
+		esReq.SeatCount = defaultSeatCount
+	}
+
 	// UpfrontFares require:
 	// * StartPlace or (StartLatitude, StartLongitude)
 	// * EndPlace	or (EndLatitude,   EndLongitude)