Fix implementation of HasBookmark to not always return true

3cd39ce refactored HasBookmark into RT::User from RT::Ticket, but in
doing so inexplicably made it into a tautology; if any one ticket was
bookmarked, all tickets passed to HasBookmark would return true.

Return to the pre-3cd39ce logic, which is correct.
basic tests for *Bookmark* methods in RT::User
Merge branch '4.2/has-bookmark'