[release-branch.go1.10] all: merge master into release-branch.go1.10
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74b56022a1 doc: note that x509 cert parsing rejects some more certs now
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c52e27e68d CONTRIBUTING: remove Pull Request bit
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829b64c1ea cmd/fix: fix cast check
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ee59f6dff2 doc: minor wording improvement to the diagnostics guide
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c6e7330ebd all: remove PULL_REQUEST_TEMPLATE from .github
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d814c2be9b doc: remove Pull Request note in README.md
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104445e314 doc: document Go 1.9.4 and Go 1.8.7
Change-Id: I58bfc6800964504258690d774a9b0aeaba509086