Positron::Handler::ArrayRef attribute hashrefs may be missing.

They were originally required to be present, even if only 'undef'.
Now they may be missing altogether. If present, though, they still
need to come directly after the tag name.

Also restructured and improved the unit test a bit.
Release v0.1.2

  Positron::Handler::ArrayRef changes:
  - Attribute HashRef no longer required to be present (but must still
    be in second position if it is)