min/mid/max_value not used for 2 and 3 color scale conditional formatting

[alex-whitney]

Description

For conditional formatting utilizing 2_color_scale or 3_color_scale formatting types, min_value, max_value, and mid_value are not used as documented.

Steps to reproduce the issue:

package main

import (
	"github.com/360EntSecGroup-Skylar/excelize"
)

func main() {
	xlsx := excelize.NewFile()
	sheet := "sheet1"

	format := `[{
		"type":"3_color_scale",
		"criteria":"=",
		"min_type":"num",
		"mid_type":"num",
		"max_type":"num",
		"min_value": "-10",
		"mid_value": "0",
		"max_value": "10",
		"min_color":"ff0000",
		"mid_color":"00ff00",
		"max_color":"0000ff"
	  }]`
	xlsx.SetConditionalFormat(sheet, "A1:A1", format)
	xlsx.SetCellValue(sheet, "A1", -5)
	xlsx.SaveAs("format.xlsx")
}

Describe the results you received:

Describe the results you expected:

Go version

$ go version
go version go1.10.3 windows/amd64

Excelize version or commit ID:

v1.3.0