Fixed #20 premature redirect on logout due to mission optional callback

[rwky]

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Is this a security patch?

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Checklist

• [x] I have read the CONTRIBUTING guidelines.
• [x] I have updated the UNRELEASED section of the CHANGELOG.
• [x] I have added myself to the contributors section of package.json if I wish to be listed
• [x] I have added test cases which verify the correct operation of this feature or patch.
• [x] I have added documentation pertaining to this feature or patch.
• [x] The automated test suite ($ npm test) executes successfully.
• [x] The automated code linting ($ npm run-script lint) executes successfully.

[coveralls]

Pull Request Test Coverage Report for Build 124

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.296%

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Totals
Change from base Build 97:	0.0%
Covered Lines:	336
Relevant Lines:	338

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💛 - Coveralls