Add spec for small Rational passed to Time::at

Author: wildmaples

spec/ruby/core/time/at_spec.rb

@@ -38,6 +38,14 @@
         Time.at(BigDecimal('1.1')).to_f.should == 1.1
       end
     end
+
+    describe "passed Rational" do
+      it "returns Time with correct nanosecond and microsecond" do
+        t = Time.at(Rational(1_486_570_508_539_759, 1_000_000))
+        t.usec.should == 539_759
+        t.nsec.should == 539_759_000
+      end
+    end
   end
 
   describe "passed Time" do