openapi: Fix response schema for PKI Issue requests

[averche]

The response schema for pki/issue/* requests have expiration represented as int64 rather than a string.

vault/builtin/logical/pki/path_issue_sign.go

Lines 418 to 421 in 922daee

	respData := map[string]interface{}{
	"expiration": int64(parsedBundle.Certificate.NotAfter.Unix()),
	"serial_number": cb.SerialNumber,
	}

This PR fixes the corresponding OpenAPI schema and should address hashicorp/vault-client-go#185

[stevendpclark]

Hi @averche, I think there's another expiration string field that should be changed to an Int64 based on this

[averche]

Hi @averche, I think there's another expiration string field that should be changed to an Int64 based on this

Good catch, thanks!

[stevendpclark]

LGTM