ACTION-258: Propose 'should' for same-party and why

Rigo: If two domains on a site want 1st party privileges (intended for 1st party use), both should be declared in the same party attribute.

Roy Fielding as best explained this issue in
http://lists.w3.org/Archives/Public/public-tracking/2013Jan/0083.html

At the end, in possibilities (y) and (z), Roy's email perfectly describes the issue and the benefit of having a first origin describing other first parties or service providers in the Tracking Status Report as "same party".
It is therefore suggested to use the following wording:
Old wording:
An optional member named same-party may be provided with an array value containing a list of domain names that the origin server claims are the same party, to the extent they are referenced by the designated resource, since all data collected via those references share the same data controller as the designated resource.

New wording:
An optional member named same-party SHOULD be provided with an array value containing a list of domain names that the origin server claims are the same party, to the extent they are referenced by the designated resource, since all data collected via those references share the same data controller as the designated resource.