Scripting: Fix regression from #1118

The new check-for-number behavior of Lua arguments broke
users who use large strings of just integers.

The Lua number check would convert the string to a number, but
that breaks user data because
Lua numbers have limited precision compared to an arbitrarily
precise number wrapped in a string.

Regression fixed and new test added.

Fixes #1118 again.
Merge pull request #1804 from mattsta/lua-regression-1118

Scripting: Fix regression from #1118