some fixes in create subscription handling and response.
added handling as per 3GPP 29.510 Chapter 6.1.6.2.16, release 17
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Attribute name: reqNotifEvents
If present, this attribute shall contain the list of event types that the NF Service Consumer is interested in receiving. If this attribute is not present, it means that notifications for all event types are requested.
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Attribute name: validityTime
Time instant after which the subscription becomes invalid. This parameter may be sent by the client, as a hint to the server, but it shall be always sent back by the server (regardless of the presence of the attribute in the request) in the response to the subscription creation request.
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exception handling in validityTime conversion.