Subscription operations corrections

**NOTE**: As per [5.4.  Subscription Description Attributes](https://tools.ietf.org/html/rfc3995#section-5.4) of [RFC 3995](https://tools.ietf.org/html/rfc3995):
>
> Subscription Description Attributes are those attributes that a
> Printer adds to a Subscription Object at the time of its creation.
>
> A Printer MUST support all attributes in this Table 2.
>
> A client MUST NOT supply the attributes in Table 2 in a Subscription
> Template Attributes Group of a Subscription Creation Operation.
> There are no corresponding default or supported attributes.
>
> Table 2 - Subscription Description Attributes
>
>      Subscription Object attributes:
>
>      notify-subscription-id (integer(1:MAX))
>      notify-sequence-number (integer(0:MAX))
>      notify-lease-expiration-time (integer(0:MAX))
>      notify-printer-up-time (integer(1:MAX))
>      notify-printer-uri (uri)
>      notify-job-id (integer(1:MAX))
>      notify-subscriber-user-name (name(MAX))