bug: change the leverage datatype of SetLeverageParams from int to float64

[cksidharthan]

Change the Leverage Params of LinearAccount -> SetLeverage Params from int to float64

From

type SetLeverageParams struct {
	Symbol           string  `url:"symbol" json:"symbol"`
	BuyLeverage  int       `url:"buy_leverage" json:"buy_leverage"`
	SellLeverage  int       `url:"sell_leverage" json:"sell_leverage"`
}

To

type SetLeverageParams struct {
	Symbol           string       `url:"symbol" json:"symbol"`
	BuyLeverage  float64    `url:"buy_leverage" json:"buy_leverage"`
	SellLeverage  float64    `url:"sell_leverage" json:"sell_leverage"`
}

[cksidharthan]

fixed as part of #29