[Bug]Funding Rate History response struct error (#544)

* Update mark_price.go

* Update unit test

Author: ChengYen-Tang

v2/futures/mark_price.go

@@ -119,7 +119,7 @@ type FundingRate struct {
 	Symbol      string `json:"symbol"`
 	FundingRate string `json:"fundingRate"`
 	FundingTime int64  `json:"fundingTime"`
-	Time        int64  `json:"time"`
+	MarkPrice   string `json:"markPrice"`
 }
 
 // GetLeverageBracketService get funding rate

v2/futures/mark_price_test.go

@@ -69,13 +69,13 @@ func (s *fundingRateServiceTestSuite) TestGetFundingRate() {
 			"symbol": "BTCUSDT",
 			"fundingRate": "-0.03750000",
 			"fundingTime": 1570608000000,
-			"time": 1576566020000
+			"markPrice": "1576566020000"
 		},
 		{
 			"symbol": "BTCUSDT",
 			"fundingRate": "0.00010000",
 			"fundingTime": 1570636800000,
-			"time": 1576566020000
+			"markPrice": "1576566020000"
 		}
 	]`)
 	s.mockDo(data, nil)
@@ -103,13 +103,13 @@ func (s *fundingRateServiceTestSuite) TestGetFundingRate() {
 			Symbol:      symbol,
 			FundingRate: "-0.03750000",
 			FundingTime: int64(1570608000000),
-			Time:        int64(1576566020000),
+			MarkPrice:   "1576566020000",
 		},
 		{
 			Symbol:      symbol,
 			FundingRate: "0.00010000",
 			FundingTime: int64(1570636800000),
-			Time:        int64(1576566020000),
+			MarkPrice:   "1576566020000",
 		},
 	}
 	s.r().Len(res, len(e))
@@ -123,7 +123,7 @@ func (s *fundingRateServiceTestSuite) assertFundingRateEqual(e, a *FundingRate)
 	r.Equal(e.Symbol, a.Symbol, "Symbol")
 	r.Equal(e.FundingRate, a.FundingRate, "FundingRate")
 	r.Equal(e.FundingTime, a.FundingTime, "FundingTime")
-	r.Equal(e.Time, a.Time, "Time")
+	r.Equal(e.MarkPrice, a.MarkPrice, "MarkPrice")
 }
 
 type getLeverageBracketServiceTestSuite struct {