Make Enumerable#each_cons return object if over size

This behavior changed in dfb47bb,
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.

Author: jeremyevans

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 @@ def test_each_slice
     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 @@ def test_each_cons
     assert_empty(ary)
 
     assert_equal(1..5, (1..5).each_cons(3) { })
+    assert_equal([], [].each_cons(3) { })
   end
 
   def test_zip