Update GetMaxBorrowableService (#669)

* Update

* Update

Author: taobig

v2/margin_service.go

@@ -957,6 +957,7 @@ func (s *ListMarginTradesService) Do(ctx context.Context, opts ...RequestOption)
 }
 
 // GetMaxBorrowableService get max borrowable of asset
+// https://developers.binance.com/docs/margin_trading/borrow-and-repay/Query-Max-Borrow
 type GetMaxBorrowableService struct {
 	c              *Client
 	asset          string
@@ -1000,7 +1001,8 @@ func (s *GetMaxBorrowableService) Do(ctx context.Context, opts ...RequestOption)
 
 // MaxBorrowable define max borrowable response
 type MaxBorrowable struct {
-	Amount string `json:"amount"`
+	Amount      string `json:"amount"`      // account's currently max borrowable amount with sufficient system availability
+	BorrowLimit string `json:"borrowLimit"` // max borrowable amount limited by the account level
 }
 
 // GetMaxTransferableService get max transferable of asset

v2/margin_service_test.go

@@ -893,7 +893,8 @@ func (s *marginTestSuite) TestListMarginTrades() {
 
 func (s *marginTestSuite) TestGetMaxBorrowable() {
 	data := []byte(`{
-		"amount": "1.69248805"
+		"amount": "1.69248805",
+		"borrowLimit": "1000"
 	}`)
 	s.mockDo(data, nil)
 	defer s.assertDo()
@@ -910,13 +911,15 @@ func (s *marginTestSuite) TestGetMaxBorrowable() {
 	r := s.r()
 	r.NoError(err)
 	e := &MaxBorrowable{
-		Amount: "1.69248805",
+		Amount:      "1.69248805",
+		BorrowLimit: "1000",
 	}
 	s.assertMaxBorrowableEqual(e, borrowable)
 }
 
 func (s *marginTestSuite) assertMaxBorrowableEqual(e, a *MaxBorrowable) {
 	s.r().Equal(e.Amount, a.Amount, "Amount")
+	s.r().Equal(e.BorrowLimit, a.BorrowLimit, "BorrowLimit")
 }
 
 func (s *marginTestSuite) TestGetMaxTransferable() {