add tests for setting metadata filters by default

[fractalwrench]

MetaData filters are currently left as null unless they are explicitly set. This adds a fix and test coverage to prevent this scenario.

Additionally I've added coverage in 2 Maze Runner scenarios: https://github.com/bugsnag/bugsnag-android/pull/262/files

[coveralls]

Pull Request Test Coverage Report for Build 1179

• 2 of 3 (66.67%) changed or added relevant lines in 2 files are covered.
• 19 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.3%) to 71.067%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
sdk/src/main/java/com/bugsnag/android/Configuration.java	0	1	0.0%

Files with Coverage Reduction	New Missed Lines	%
sdk/src/main/java/com/bugsnag/android/MetaData.java	1	93.0%
sdk/src/main/java/com/bugsnag/android/DeviceData.java	2	77.44%
sdk/src/main/java/com/bugsnag/android/Client.java	3	53.71%
sdk/src/main/java/com/bugsnag/android/SessionTracker.java	13	54.62%

Totals
Change from base Build 1155:	0.3%
Covered Lines:	1771
Relevant Lines:	2492

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💛 - Coveralls

[snmaynard]

Why not just set the default in the metadata class?

[fractalwrench]

You're right, that's a much better way of doing it - I've updated the PR.