How to make the first letter of a string lowercase in Go Language?

How to make the first letter of a string lowercase, but not change the case of any of the other letters?

For example: 

"Go language is awesome" → "go language is awesome"
"Hello world" → "hello world"

Solution 1:

package main

import (
    "unicode"
    "unicode/utf8"
)

// Main function
func main() {

    str := "Go language is awesome"
    println(firstToLower(str))
}

func firstToLower(s string) string {
    if len(s) > 0 {
        r, size := utf8.DecodeRuneInString(s)
        if r != utf8.RuneError || size > 1 {
            lower := unicode.ToLower(r)
            if lower != r {
                s = string(lower) + s[size:]
            }
        }
    }
    return s
}

Output:

Solution 2: Easy

package main

import "strings"

// Main function
func main() {

    str := "Go language is awesome"
    str = strings.ToLower(string(str[0])) + str[1:]
    println(str)
}

Output:

Solution 3: Normal

package main

import (
    "unicode"
)

// Main function
func main() {

    str := "Go language is awesome"
    runes := []rune(str)
    if len(runes) > 0 {
        runes[0] = unicode.ToLower(runes[0])
    }
    println(string(runes))
}

Output: