Rand fixes

Proposed fixes for https://github.com/rubinius/rubinius/issues/3180

Does not completely fix case 3 as there is weird type coercion that goes deeper: https://github.com/rubinius/rubinius/issues/3181
We don't want random `require` calls in specs. This should either be moved to the top of the file, or the whole test should be moved into its own file. 

@brixen Any thoughts on this, is this something we perhaps want in rubysl-bigdecimal instead?