fix args sent to shutdown

[jbarz1]

fixes following error when submitting a cancellation message.

s3transfer/futures.py", line 279, in cancel
self._exception = exc_type(msg)
TypeError: 'str' object is not callable

[codecov-commenter]

Codecov Report

Base: 85.32% // Head: 85.32% // No change to project coverage :thumbsup:

Coverage data is based on head (16ee9a3) compared to base (7a0d980). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #255 +/- ## ======================================== Coverage 85.32% 85.32% ======================================== Files 16 16 Lines 2713 2713 ======================================== Hits 2315 2315 Misses 398 398 ``` | [Impacted Files](https://codecov.io/gh/boto/s3transfer/pull/255?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boto) | Coverage Δ | | |---|---|---| | [s3transfer/manager.py](https://codecov.io/gh/boto/s3transfer/pull/255/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boto#diff-czN0cmFuc2Zlci9tYW5hZ2VyLnB5) | `96.55% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boto). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boto)

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[cfxegbert]

See pull request #253 for integration test as well as the fix