Reduce Duration#inspect to a single series of transformations

* eliminates need for temp Hash

Also added a couple of examples to DurationTest to specify:

* duration can be defined with units out of order e.g.
  1.month + 1.year + 1.second + 1.day
* equality with a Fixnum works regardless of which operand is on which
  side of the operator
Merge pull request #12200 from dchelimsky/simplify-duration-inspect-even-more

Reduce Duration#inspect to a single series of transformations