RFR: args: fix toString() for trailing input

[Erik Helin]

Hi all,

please review this small patch that fixes `Input.toString` for trailing input. I also added an additional unit test
that tests `Input.toString`.

Testing:
- [x] Added a new unit test
- [x] `make test` passes on Linux x64

Thanks,
Erik

-------------

Commit messages:
 - args: fix toString() for trailing input

Changes: https://git.openjdk.java.net/skara/pull/698/files
 Webrev: https://webrevs.openjdk.java.net/skara/698/webrev.00
  Stats: 68 lines in 2 files changed: 66 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/698.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/698/head:pull/698

PR: https://git.openjdk.java.net/skara/pull/698