Sort Signing Keys on Marshal

[sethjback]

Sort the signing keys by their public nkey during the json marshaling process to ensure the generated json array is always the same.

Since ranging over a map in golang is intentionally random, prior behavior would generate a random order for the signing key output. Arrays in json are considered ordered, so this would cause test that compare the json generated from the same SigningKeys to occasionally fail: ["sk1","sk2"] != ["sk2","sk1"]

[coveralls]

Pull Request Test Coverage Report for Build 5930771495

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 80.957%

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Totals
Change from base Build 5928958397:	0.02%
Covered Lines:	2691
Relevant Lines:	3324

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💛 - Coveralls

[aricart]

Note that JWTs are not only generated by the go library, so this behaviour is not going to hold for anything that comes from the outside. I would instead guarantee the ordering you want as application logic on decoding. With that said, I like the presentation to be ordered as well.

[aricart]

LGTM