Make Enumerable#each_cons return object if over size

This behavior changed in dfb47bbd17,
but only for normal exit, not for early exit.  Fix it for early
exit as well.

While here, fix example code in documentation so that it doesn't
indicate that the method returns nil.

enum.c

@@ -2968,12 +2968,12 @@ enum_each_slice_size(VALUE obj, VALUE args, VALUE eobj)
  *  returns +self+:
  *
  *    a = []
- *    (1..10).each_slice(3) {|tuple| a.push(tuple) } # => nil
+ *    (1..10).each_slice(3) {|tuple| a.push(tuple) }
  *    a # => [[1, 2, 3], [4, 5, 6], [7, 8, 9], [10]]
  *
  *    a = []
  *    h = {foo: 0, bar: 1, baz: 2, bat: 3, bam: 4}
- *    h.each_slice(2) {|tuple| a.push(tuple) } # => nil
+ *    h.each_slice(2) {|tuple| a.push(tuple) }
  *    a # => [[[:foo, 0], [:bar, 1]], [[:baz, 2], [:bat, 3]], [[:bam, 4]]]
  *
  *  With no block given, returns an Enumerator.
@@ -3047,12 +3047,12 @@ enum_each_cons_size(VALUE obj, VALUE args, VALUE eobj)
  *  returns +self+:
  *
  *    a = []
- *    (1..5).each_cons(3) {|element| a.push(element) } # => nil
+ *    (1..5).each_cons(3) {|element| a.push(element) }
  *    a # => [[1, 2, 3], [2, 3, 4], [3, 4, 5]]
  *
  *    a = []
  *    h = {foo: 0,  bar: 1, baz: 2, bam: 3}
- *    h.each_cons(2) {|element| a.push(element) } # => nil
+ *    h.each_cons(2) {|element| a.push(element) }
  *    a # => [[[:foo, 0], [:bar, 1]], [[:bar, 1], [:baz, 2]], [[:baz, 2], [:bam, 3]]]
  *
  *  With no block given, returns an Enumerator.
@@ -3068,7 +3068,7 @@ enum_each_cons(VALUE obj, VALUE n)
     if (size <= 0) rb_raise(rb_eArgError, "invalid size");
     RETURN_SIZED_ENUMERATOR(obj, 1, &n, enum_each_cons_size);
     arity = rb_block_arity();
-    if (enum_size_over_p(obj, size)) return Qnil;
+    if (enum_size_over_p(obj, size)) return obj;
     memo = MEMO_NEW(rb_ary_new2(size), dont_recycle_block_arg(arity), size);
     rb_block_call(obj, id_each, 0, 0, each_cons_i, (VALUE)memo);
 

test/ruby/test_enum.rb

@@ -739,6 +739,7 @@ class TestEnumerable < Test::Unit::TestCase
     assert_equal([[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]], ary)
 
     assert_equal(1..10, (1..10).each_slice(3) { })
+    assert_equal([], [].each_slice(3) { })
   end
 
   def test_each_cons
@@ -760,6 +761,7 @@ class TestEnumerable < Test::Unit::TestCase
     assert_empty(ary)
 
     assert_equal(1..5, (1..5).each_cons(3) { })
+    assert_equal([], [].each_cons(3) { })
   end
 
   def test_zip