Fix unintended change of Array which is passed as argument

## Problem

`mlcs(arr)` deletes 1st member of `arr`


## Example

```coffee
arr = ["123", "12345", "123456"]
mlcs = require "mlcs"
console.log arr # =>  ["123", "12345", "123456"]
mlcs arr           # => '123'
console.log arr # => ["12345", "123456"]  "123" is deleted !!
```
This PR is for fixing this problem :smiley: