Make Range#step to consistently use + for iteration [Feature #18368]

Previously, non-numerics expected step to be integer,
and iterated with begin#succ, skipping over step value
steps. Since this commit, numeric and non-numeric iteration
behaves the same way, by using + operator.

Depending on the float range, there could be an off-by-one error,
where the last result that should be in the range was missed. Fix
this by checking if the computed value for the expected value
outside the range is still inside the range, and if so, increment
the step size.

Fixes [Bug #16612]