Fix for Rational objects in Time::at

The fix is to reassign the sec and usec arguments in the case of
a Rational sec argument by splitting the Rational into numerator and
denominator. This creates the same behaviour as
Time::at(seconds, microseconds_with_frac).

Author: wildmaples

src/main/ruby/truffleruby/core/time.rb

@@ -336,7 +336,17 @@ def at(sec, usec=undefined, unit=undefined, **kwargs)
           raise ArgumentError, "unexpected unit: #{unit}"
         end
 
-      usec = 0 if Primitive.undefined?(usec)
+      if Primitive.undefined?(usec) && Primitive.object_kind_of?(sec, Rational)
+        rational = Truffle::Type.coerce_to_exact_num(sec)
+        seconds, fractional_seconds = rational.divmod(1)
+        nano_fractional_seconds = fractional_seconds * 1_000_000_000
+
+        time = Primitive.time_at self, seconds.to_i, nano_fractional_seconds.to_i
+        time = Primitive.time_localtime(time, offset) if offset
+        return time
+      elsif Primitive.undefined?(usec)
+        usec = 0
+      end
 
       s = Truffle::Type.coerce_to_exact_num(sec)
       u = Truffle::Type.coerce_to_exact_num(usec)