Lowercase CN in all code paths (#6824)

When returning a CN from csr.NamesFromCSR, ensure that we call
strings.ToLower on that name in all return paths. This prevents us from
running into lint failures (and therefore refusing to issue) when an
applicant submits a CSR containing uppercase SANs and no explicit CN.

csr/csr.go

@@ -123,7 +123,7 @@ func NamesFromCSR(csr *x509.CertificateRequest) names {
 	// which is shorter than the the maximum acceptable CN length (if any).
 	for _, name := range sans {
 		if len(name) <= maxCNLength {
-			return names{SANs: core.UniqueLowerNames(sans), CN: name}
+			return names{SANs: core.UniqueLowerNames(sans), CN: strings.ToLower(name)}
 		}
 	}
 

csr/csr_test.go

@@ -184,6 +184,14 @@ func TestNamesFromCSR(t *testing.T) {
 			"a.com",
 			[]string{"a.com"},
 		},
+		{
+			"no explicit CN, uppercase SAN",
+			&x509.CertificateRequest{DNSNames: []string{
+				"A.com",
+			}},
+			"a.com",
+			[]string{"a.com"},
+		},
 		{
 			"no explicit CN, too long leading SANs",
 			&x509.CertificateRequest{DNSNames: []string{