[BUG] if string is a constant expr not containing any variable, the result is wrong

[wangzhanbing]

if string is a constant expr not containing any variable, the result is wrong
for example

rawYQL = "3>1"
result, err = yql.Match(rawYQL, map[string]interface{}{})
fmt.Println(rawYQL, "is ", result, "err: ", err)

# Output: 3>1 is  false err:  <nil>

expect result is true

[wangzhanbing]

Do you have expected time fixing this bug ?

otherwise, I think we can add a parameter to config whether to ignore upper or lower in expr