Upgrade from 1.x fails if any docID starts with "_"

Via this forum thread — apparently it was somehow possible in 1.x to create a document whose ID started with "_", even though this is supposed to be illegal. Any database with such a doc will fail to open in 2.x because the upgrader will reject that docID.

We have a test in the Upgrader class to skip the special case of docIDs that start with "design/" (which was legal in 1.x, for complicated reasons.) This should be modified to skip (not reject) any doc whose ID starts with "".