fix some problems: (#590)

* fix some problems:1.fix the path of asset_index_service.go and
constituents_service.go. 2.fix struct DeliveryPrice. 3.remove redundant
funding_rate_history.go

* remove responding interface in client.go

Author: xyq-c-cpp

v2/futures/asset_index_service.go

@@ -33,7 +33,7 @@ func (s *AssetIndexService) Symbol(symbol string) *AssetIndexService {
 func (s *AssetIndexService) Do(ctx context.Context, opts ...RequestOption) (res []*AssetIndex, err error) {
 	r := &request{
 		method:   http.MethodGet,
-		endpoint: "/futures/v1/assetIndex",
+		endpoint: "/fapi/v1/assetIndex",
 	}
 	if s.symbol != nil {
 		r.setParam("symbol", *s.symbol)

v2/futures/client.go

@@ -556,11 +556,6 @@ func (c *Client) NewFundingRateService() *FundingRateService {
 	return &FundingRateService{c: c}
 }
 
-// NewFundingRateHistoryService init funding rate history service
-func (c *Client) NewFundingRateHistoryService() *FundingRateHistoryService {
-	return &FundingRateHistoryService{c: c}
-}
-
 // NewFundingRateInfoService init funding rate info service
 func (c *Client) NewFundingRateInfoService() *FundingRateInfoService {
 	return &FundingRateInfoService{c: c}

v2/futures/constituents_service.go

@@ -30,7 +30,7 @@ func (s *ConstituentsService) Symbol(symbol string) *ConstituentsService {
 func (s *ConstituentsService) Do(ctx context.Context, opts ...RequestOption) (res *ConstituentsServiceRsp, err error) {
 	r := &request{
 		method:   http.MethodGet,
-		endpoint: "/futures/v1/assetIndex",
+		endpoint: "/fapi/v1/constituents",
 	}
 	r.setParam("symbol", s.symbol)
 

v2/futures/data_service.go

@@ -12,8 +12,8 @@ type DeliveryPriceService struct {
 }
 
 type DeliveryPrice struct {
-	DeliveryTime  uint64  `json:"DeliveryTime"`  // deliveryTime
-	DeliveryPrice float64 `json:"DeliveryPrice"` // deliveryPrice
+	DeliveryTime  uint64  `json:"deliveryTime"`  // deliveryTime
+	DeliveryPrice float64 `json:"deliveryPrice"` // deliveryPrice
 }
 
 func (s *DeliveryPriceService) Pair(pair string) *DeliveryPriceService {