Fix missing app.config.is_set() in mock config for dry-run

[natefoo]

Fixes #71.

I can't say that True will always be the right answer but it works for the specific issue encountered.

[coveralls]

Pull Request Test Coverage Report for Build 4177992766

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 96.692%

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Totals
Change from base Build 4141797386:	-0.2%
Covered Lines:	929
Relevant Lines:	952

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💛 - Coveralls