Fix method override

[wesselb]

Fixes method overrides for equivalent but not identical signatures.

Also removes duplicate tests and fixes the faithfulness check for new unions.

Also adds a notice to the docs about mixed-type containers.

Fixes #214.

[coveralls]

Pull Request Test Coverage Report for Build 18139335504

Details

• 7 of 7 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 99.612%

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Totals
Change from base Build 18126645549:	0.002%
Covered Lines:	1283
Relevant Lines:	1288

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💛 - Coveralls