Fix warning in MorboDB::Cursor when calling sort() with a hashref

- sort() was using Scalar::Util::blessed to check if its
  argument was a Tie::IxHash:

      if ($order && blessed $order eq 'Tie::IxHash') { ... }

  However, blessed() returns undef if its argument isn't a
  blessed reference (unlike the ref builtin which returns an
  empty string for non-references). Changing it to use ref()
  avoids the warning.

- Added a new test to demonstrate the warning.
Saving a document should perform an insert if there's no _id

- If you save a document (via MorboDB::Collection::save)
  that has no _id field, it should perform an insert, adding
  a unique OID to the document. This is for consistency with
  MongoDB (and is actually the documented behavoir in the
  MorboDB POD.)

- Added some new test cases for save() for both manual and
  automatic OIDs.