The current implementation serializes zero-length durations incorrectly (it serializes as `"-P"`), and cannot un-serialize itself:
```
[1] pry(main)> ActiveSupport::Duration.parse(0.minutes.iso8601)
ActiveSupport::Duration::ISO8601Parser::ParsingError: Invalid ISO 8601 duration: "-P" is empty duration
from /Users/rando/.gem/ruby/2.3.1/gems/activesupport-5.0.0/lib/active_support/duration/iso8601_parser.rb:96:in `raise_parsing_error'
```
Postgres empty intervals are serialized as `"PT0S"`, which is also parseable by the Duration deserializer, so I've modified the `ISO8601Serializer` to do the same.
Additionally, the `#normalize` function returned a negative sign if `parts` was blank (all zero). Even though this fix does not rely on the sign, I've gone ahead and corrected that, too, in case a future refactoring of `#serialize` uses it.
```ruby
ActiveSupport::Duration.parse('P3Y6M4DT12H30M5S')
(3.years + 3.days).iso8601
```
Inspired by Arnau Siches' [ISO8601 gem](https://github.com/arnau/ISO8601/)
and rewritten by Andrey Novikov with suggestions from Andrew White. Test
data from the ISO8601 gem redistributed under MIT license.
(Will be used to support the PostgreSQL interval data type.)
