Update to 1.21
- New upstream release 1.21
- Declare variables with our() to avoid warnings (GH#109, GH#171)
- Empty mapping value at the end resolves to null (was becoming empty string)
(GH#131, GH#170)
- Output key in warning when duplicate key was found (GH#119, GH#169)
- Allow reading and writing to IO::Handle (GH#157, GH#168)