Suppress keyword argument warning from #step

* numeric.c (num_step): pass the extracted argument from keyword
  argument, not the last argument itself which should have been
  warned already.

numeric.c

@@ -2768,7 +2768,7 @@ num_step(int argc, VALUE *argv, VALUE from)
                                     num_step_size, from, to, step, FALSE);
         }
 
-        RETURN_SIZED_ENUMERATOR(from, argc, argv, num_step_size);
+        return SIZED_ENUMERATOR(from, 2, ((VALUE [2]){to, step}), num_step_size);
     }
 
     desc = num_step_scan_args(argc, argv, &to, &step, TRUE, FALSE);

test/ruby/test_numeric.rb

@@ -292,6 +292,14 @@ class TestNumeric < Test::Unit::TestCase
     assert_raise(ArgumentError, bug9811) { 1.step(10, 1, by: 11) {} }
     assert_raise(ArgumentError, bug9811) { 1.step(10, 1, by: 11).size }
 
+
+    e = assert_warn(/The last argument is used as the keyword parameter/) {
+      1.step(10, {by: "1"})
+    }
+    assert_warn('') {
+      assert_raise(ArgumentError) {e.size}
+    }
+
     assert_equal(bignum*2+1, (-bignum).step(bignum, 1).size)
     assert_equal(bignum*2, (-bignum).step(bignum-1, 1).size)