fix the code of custom samplers/updaters

[segasai]

original patch by @ColmTalbot rewritten test by @segasai

[coveralls]

Pull Request Test Coverage Report for Build 3213912432

• 6 of 6 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 91.472%

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Totals
Change from base Build 3192347414:	0.1%
Covered Lines:	3990
Relevant Lines:	4362

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💛 - Coveralls

[ColmTalbot]

Thanks! and sorry for not patching the tests promptly, it was a busy few days.