Checks if DiagonalGate has modulus 1

[1ucian0]

Fixes #3842

I took the suggested solution from #3842 (thanks @hhyap!) and the test from #4375 (thanks @ewinston!)

This issue was simple enough if you do not consider the full extend of #4375

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[coveralls]

Pull Request Test Coverage Report for Build 2370777544

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 84.376%

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Totals
Change from base Build 2370437301:	0.002%
Covered Lines:	54539
Relevant Lines:	64638

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💛 - Coveralls

[jakelishman]

Looks good, thanks!