Support Map in SetArg

[lovung]

Requested feature

• Like this PR https://github.com/golang/mock/pull/98, I hope you can support to SetArg for the Map.

Why the feature is needed

• In the most case, Golang use Map as an argument of the function, not pointer of Map. But inside the function, we can still Add/Delete entry in this map, but can not initial if the input map is nil. So, we need to check if this Argument is nil or not.

Example, I want to generate the Mock for AddItem function and use SetArg to set the after value.

https://play.golang.org/p/sXnsc8FExox

package main

import (
    "fmt"
)

func main() {
    var exampleMap = make(map[string]string)
    AddItem(exampleMap, "A", "B")
    fmt.Println(exampleMap)
}

func AddItem(input map[string]string, key, value string) {
    if input == nil {
        return
    }
    input[key] = value
}