Changed default argument value for printer_uri to colander.null

As stated in [RFC 2911, section 3.3.4](http://tools.ietf.org/search/rfc2911#section-3.3.4): 

> 3.3.4.1 Get-Job-Attributes Request
>
>   The following groups of attributes are part of the Get-Job-Attributes
>   Request when the request is directed at a Job object:
>
>   Group 1: Operation Attributes
>
>      Target:
>        Either (1) the "printer-uri" (uri) plus "job-id"
>        (integer(1:MAX)) or (2) the "job-uri" (uri) operation
>        attribute(s) which define the target for this operation as
>        described in section 3.1.5.

one may pass either the `job-uri` **OR** (`printer-uri` **AND** `job-id`), in order to fetch the job attributes ... so we're allowing a `colander.null` value for the `printer_uri`, which makes it optional, and when not received, will not be included in the request.