Fix regressions in pl.strict

This pull request fixes regressions in pl.strict.

Assigning nil to a global should declare it, as documented in the test file and [here](https://github.com/catwell/Penlight/blob/acf90667a84d3f7294d9aa67cb67a57be8987bc0/lua/pl/strict.lua#L2-L3). The first commit fixes this and adds a test.

Read access to undeclared globals used to be allowed from C, the second commit restores this behavior. Not having it breaks, among other things, the interpreter:

```
$ lua -lpl.strict
Lua 5.2.1  Copyright (C) 1994-2012 Lua.org, PUC-Rio
> repeat
variable '_PROMPT2' is not declared
```