Skip calling pkix_decode_cert if the certs are already of tuple types

[pvthuyen]

• Resolves https://github.com/elixir-mint/mint/issues/377

• :public_key.cacerts_get/0 returns combined_cert which contains both the undecoded binary and the decoded #'OTPCertificate'{}

  • While calling :public_key.pkix_decode_cert with :plain returns #'Certificate'{}, both #'OTPCertificate'{} and #'Certificate'{} share very similar structure (document) and they both have subjectPublicKeyInfo and validity. Since we only depends on those 2 data to verify the certificates, just passing the #'OTPCertificate'{} from :public_key.cacerts_get/0 through should be sufficient

[coveralls]

Pull Request Test Coverage Report for Build 8f66a7f82d236b11b68fad4b92f7fc9a51824fbb-PR-382

• 2 of 3 (66.67%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.1%) to 84.942%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/mint/core/transport/ssl.ex	2	3	66.67%
<!--	Total:	2	3	66.67%	-->

Files with Coverage Reduction	New Missed Lines	%
test/support/mint/http1/test_server.ex	1	84.62%
<!--	Total:	1		-->

Totals
Change from base Build 6b932144282495b5154a4f6f0db456f96c4ec363:	-0.1%
Covered Lines:	1320
Relevant Lines:	1554

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💛 - Coveralls

[ericmj]

Thank you @pvthuyen. I added a test and updated the docs also.