Set supports_pending_changesets for Plastic.
Set supports_pending_changesets for Plastic.

When I added the supports_pending_changesets flag, I neglected to add it for
Plastic. This broke things.

Fixes part of bug 3044.

chipx86/reviewboard,reviewboard/reviewboard,sgallagher/reviewboard,reviewboard/reviewboard,KnowNo/reviewboard,beol/reviewboard,bkochendorfer/reviewboard,1tush/reviewboard,1tush/reviewboard,1tush/reviewboard,custode/reviewboard,1tush/reviewboard,custode/reviewboard,sgallagher/reviewboard,1tush/reviewboard,brennie/reviewboard,brennie/reviewboard,1tush/reviewboard,bkochendorfer/reviewboard,davidt/reviewboard,sgallagher/reviewboard,1tush/reviewboard,KnowNo/reviewboard,custode/reviewboard,bkochendorfer/reviewboard,chipx86/reviewboard,sgallagher/reviewboard,KnowNo/reviewboard,custode/reviewboard,chipx86/reviewboard,beol/reviewboard,beol/reviewboard,bkochendorfer/reviewboard,brennie/reviewboard,1tush/reviewboard,davidt/reviewboard,chipx86/reviewboard,brennie/reviewboard,beol/reviewboard,1tush/reviewboard,davidt/reviewboard,KnowNo/reviewboard,davidt/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard