rational.c: convert a numerator to rational before calling fdiv in Kernel.Rational() (#3702)

This makes `Rational(BigDecimal(1), 60) == Rational(1, 60)`.
[Bug #16518]

rational.c

@@ -2673,8 +2673,16 @@ nurat_convert(VALUE klass, VALUE numv, VALUE denv, int raise)
             }
         }
         if ((k_numeric_p(a1) && k_numeric_p(a2)) &&
-                (!f_integer_p(a1) || !f_integer_p(a2)))
+                (!f_integer_p(a1) || !f_integer_p(a2))) {
+            VALUE tmp = rb_protect(to_rational, a1, &state);
+            if (!state) {
+                a1 = tmp;
+            }
+            else {
+                rb_set_errinfo(Qnil);
+            }
             return f_div(a1, a2);
+        }
     }
 
     a1 = nurat_int_value(a1);

test/ruby/test_rational.rb

@@ -165,6 +165,14 @@ class Rational_Test < Test::Unit::TestCase
     if (1.0/0).infinite?
       assert_raise(FloatDomainError){Rational(1.0/0)}
     end
+
+    bug16518 = "[ruby-core:96942] [Bug #16518]"
+    cls = Class.new(Numeric) do
+      def /(y); 42; end
+      def to_r; 1r; end
+      def to_int; 1; end
+    end
+    assert_equal(1/2r, Rational(cls.new, 2))
   end
 
   def test_attr