[BUG] candidate_expression was actually being generated incorrectly

somewhere in the evals, the '==' is being ditched to a =.  We're also losing the .to_s part of the removable drives test.  This used to work in chef <11, but since it's technically an attribute assignment chef11 doesn't allow this any more (and rightly so).

This is a fairly hideous regex sub to fix the expression up again before it is run.  probably need to fix the part where it's going wrong in the first place, but this works.