different behavior in io.popen function

[0x501D]

In the C implementation of the Lua interpreter, in the io.popen function the second argument can be nil:

Lua 5.1.5 Copyright (C) 1994-2012 Lua.org, PUC-Rio:

> x,y = io.popen("ls", nil)
> assert(x)
> assert(y == nil)
> 

Gopher lua throws an exception.
gopher-lua (current master):

bad argument #2 to popen (string expected, got nil)

affected public project