Fix remaining 1.9 String#% RubySpec by making "%u" an alias of "%d"

Also fix two other failing RubySpecs for Kernel#sprintf that were
affected by the change.
Add RubySpec for Kernel#sprintf and the "%d" format
Merge pull request #2265 from leocassarani/sprinter-19-ints

Make "%u" an alias of "%d" on 1.9