UWS: incorrect representation of the quote endpoint

[vforchi]

Currently the endpoint quote returns the value of the field quote in the class UWSJob, with -1 meaning no quote, but according to UWS it should return either the empty string or an ISO8601 date:

https://www.ivoa.net/documents/UWS/20161024/REC-UWS-1.1-20161024.html#Quote

/{jobs}/{job-id}/quote	the Quote for {job-id}	[std:iso8601] If no quote available then return empty string.