Added test for bug #8485 (MRI 1.9.3-2.0)

Date objects with same Julian day should be equivalent. For some ranges,
Date.civil in 1.9.3 creates objects with non-compliant semantics.
Merge pull request #221 from teleological/ruby_bug_8485

Added test for bug #8485 (MRI 1.9.3-2.0)