Make product consistently yield an array of N elements instead of N arguments

Inconsistency pointed out by @mame:

```
>> Enumerator.product([1], [2], [3]).to_a
=> [[1, 2, 3]]
>> Enumerator.product([1], [2]).to_a
=> [[1, 2]]
>> Enumerator.product([1]).to_a
=> [1]
>> Enumerator.product().to_a
=> [nil]
```

Got fixed as follows:

```
>> Enumerator.product([1], [2], [3]).to_a
=> [[1, 2, 3]]
>> Enumerator.product([1], [2]).to_a
=> [[1, 2]]
>> Enumerator.product([1]).to_a
=> [[1]]
>> Enumerator.product().to_a
=> [[]]
```

This was due to the nature of the N-argument funcall in Ruby.

enumerator.c

@@ -3434,7 +3434,7 @@ enumerator_plus(VALUE obj, VALUE eobj)
  *
  * The method used against each enumerable object is `each_entry`
  * instead of `each` so that the product of N enumerable objects
- * yields exactly N arguments in each iteration.
+ * yields an array of exactly N elements in each iteration.
  *
  * When no enumerator is given, it calls a given block once yielding
  * an empty argument list.
@@ -3627,7 +3627,7 @@ product_each(VALUE obj, struct product_state *pstate)
         rb_block_call(eobj, id_each_entry, 0, NULL, product_each_i, (VALUE)pstate);
     }
     else {
-        rb_funcallv(pstate->block, id_call, pstate->argc, pstate->argv);
+        rb_funcall(pstate->block, id_call, 1, rb_ary_new_from_values(pstate->argc, pstate->argv));
     }
 
     return obj;

test/ruby/test_enumerator.rb

@@ -908,46 +908,82 @@ class TestEnumerator < Test::Unit::TestCase
   end
 
   def test_product
+    ##
+    ## Enumerator::Product
+    ##
+
+    # 0-dimensional
     e = Enumerator::Product.new
     assert_instance_of(Enumerator::Product, e)
     assert_kind_of(Enumerator, e)
     assert_equal(1, e.size)
     elts = []
-    e.each { |*x| elts << x }
+    e.each { |x| elts << x }
     assert_equal [[]], elts
+    assert_equal elts, e.to_a
+    heads = []
+    e.each { |x,| heads << x }
+    assert_equal [nil], heads
 
+    # 1-dimensional
+    e = Enumerator::Product.new(1..3)
+    assert_instance_of(Enumerator::Product, e)
+    assert_kind_of(Enumerator, e)
+    assert_equal(3, e.size)
+    elts = []
+    e.each { |x| elts << x }
+    assert_equal [[1], [2], [3]], elts
+    assert_equal elts, e.to_a
+
+    # 2-dimensional
     e = Enumerator::Product.new(1..3, %w[a b])
     assert_instance_of(Enumerator::Product, e)
     assert_kind_of(Enumerator, e)
     assert_equal(3 * 2, e.size)
     elts = []
-    e.each { |*x| elts << x }
+    e.each { |x| elts << x }
     assert_equal [[1, "a"], [1, "b"], [2, "a"], [2, "b"], [3, "a"], [3, "b"]], elts
+    assert_equal elts, e.to_a
+    heads = []
+    e.each { |x,| heads << x }
+    assert_equal [1, 1, 2, 2, 3, 3], heads
 
+    # Reject keyword arguments
     assert_raise(ArgumentError) {
       Enumerator::Product.new(1..3, foo: 1, bar: 2)
     }
 
+    ##
+    ## Enumerator.product
+    ##
+
+    # without a block
     e = Enumerator.product(1..3, %w[a b])
     assert_instance_of(Enumerator::Product, e)
 
+    # with a block
     elts = []
-    ret = Enumerator.product(1..3, %w[a b]) { |*x| elts << x }
+    ret = Enumerator.product(1..3) { |x| elts << x }
     assert_instance_of(Enumerator::Product, ret)
-    assert_equal [[1, "a"], [1, "b"], [2, "a"], [2, "b"], [3, "a"], [3, "b"]], elts
+    assert_equal [[1], [2], [3]], elts
+    assert_equal elts, Enumerator.product(1..3).to_a
 
+    # an infinite enumerator and a finite enumerable
     e = Enumerator.product(1.., 'a'..'c')
     assert_equal(Float::INFINITY, e.size)
     assert_equal [[1, "a"], [1, "b"], [1, "c"], [2, "a"]], e.take(4)
 
+    # an infinite enumerator and an unknown enumerator
     e = Enumerator.product(1.., Enumerator.new { |y| y << 'a' << 'b' })
     assert_equal(Float::INFINITY, e.size)
     assert_equal [[1, "a"], [1, "b"], [2, "a"], [2, "b"]], e.take(4)
 
+    # an infinite enumerator and an unknown enumerator
     e = Enumerator.product(1..3, Enumerator.new { |y| y << 'a' << 'b' })
     assert_equal(nil, e.size)
     assert_equal [[1, "a"], [1, "b"], [2, "a"], [2, "b"]], e.take(4)
 
+    # Reject keyword arguments
     assert_raise(ArgumentError) {
       Enumerator.product(1..3, foo: 1, bar: 2)
     }