Fix varargs function call with array

Call varargs function with an array as last argument act now like in Java,
so that the following code has expected behaviour :

```golo
  function varargs = |args...| {
    foreach (arg in args) {
      println(arg)
    }
  }

  function main = |args...| {
    let a = array[1, 2, 3, 4]
    varargs(a)
    # prints :
    # 1
    # 2
    # 3
    # 4
  }
```

Call a varags function with an empty varargs argument caused a NoSuchMethodError,
so that the following code is now correct :

```golo
  function varargs = |str,args...| {
    println(str)
    foreach (arg in args) {
      println(arg)
    }
  }

  function main = |args...| {
    varargs("foo")
    # prints :
    # foo
  }
```

This two changes are done for closures and constructors too.